A method to direct differentiation of pluripotent stem cells into functional heart muscle

ABSTRACT

The present invention is directed to a method for producing bioengineered heart muscle (BHM) from pluripotent stem cells, generally comprising the steps of inducing mesoderm differentiation, cardiac differentiation, and cardiac maturation by directed tissue formation. The method is a robust, serum-free and reproducible way to produce BHM for multiple applications, and is applicable to multiple pluripotent stem cell lines. The present invention is also directed to the BHM produced by the method disclosed herein, as well as to uses of said BHM in pharmacologic and toxicity screenings, and its use in medicine.

BACKGROUND OF THE INVENTION

Human pluripotent stem cells (hPSCs) are now widely used to provide atheoretically endless and also large supply of human cardiomyocytes(Kehat et al. J Clin Invest 108, 407-414 (2001); Takahashi et al. Cell131, 861-872 (2007); Zhang et al., Circ Res 104, e30-41 (2009)). Humancardiomyocytes have been derived from human embryonic stem cells (hESCs)(Thomson et al. Science 282, 1145-1147 (1998)) and induced pluripotentstem cells (hIPSCs) (Takahashi et al., Cell 131, 861-872 (2007)) andhave a demonstrated use for multiple purposes including developmentalmodels (Lian et al. Stem Cells 2012 (2012)), drug efficacy and/or safetyscreening (Schaaf et al. PLoS ONE 6, 20 (2011)), hypertrophy modellingand regenerative applications. Additionally, with recent advances inhIPSC technology, cardiomyocytes exhibiting heritable genetic diseasephenotypes can be generated in vitro (Carvajal-Vergara, X. et al. Nature465, 808-812 (2010); Itzhaki et al., Nature 471, 225-229 (2011); Malanet al. Circ Res (2011); Moretti et al., N Engl J Med 363, 1397-1409(2010); Yazawa et al. Nature 471, 230-234 (2010)).

It is now widely accepted that the low density 2D culture ofbiopsy-derived human cardiomyocytes leads to rapid changes incardiomyocyte phenotype and morphology (Bird et al. Cardiovasc Res 58,423-434 (2003)) and making it difficult to extrapolate results to the invivo situation. In order to obtain a cardiomyocyte phenotype morerepresentative of in vivo conditions, cardiac tissue engineering hasbeen used (Eschenhagen et al. FASEB J 11, 683-694 (1997); Zimmermann etal. Biotechnol Bioeng 68, 106-114 (2000), Zimmermann et al. Circ Res 90,223-230 (2002); Tulloch et al. Circ Res 109, 47-59 (2011); Tiburcy etal. Circ Res 109, 1105-1114 (2011); Eschenhagen et al. Am J PhysiolHeart Circ Physiol 303, 11 (2012)) to generate constructs with similarproperties to the native heart tissue.

The current ideology of tissue engineering is to generate/isolate therequired cell type(s), and seed them into an engineered environment topromote their differentiation and generate in vivo-like tissues. Tissueengineering may therefore be considered as an inefficient process fortwo reasons, 1) disassociation of a tissue/differentiation culturedestroys the extracellular environment thus destroying developmentalinformation (eg. cell-cell interconnectivity, geometric cellpositioning, cell-ECM connectivity), this necessitates very largeincreases in extracellular matrix (ECM) production in order to re-buildthe environment (Hudson et al. Tissue Eng Part A 17, 2279-2289 (2011)),and 2) the disassociation process is variable between hPSC lines and canlead to considerable cell death.

Other protocols reported in the literature may require modification ofthe protocol to enable similar cardiomyocyte efficiencies in multiplehPSC lines. However, the inventor's results demonstrate that changes indifferentiation protocol may greatly affect the cardiomyocyte phenotype(e.g. it is shown that dorsomorphin may greatly affect the bioengineeredheart muscle (BHM)). This may lead to changes in tissue engineeredmyocardial properties which may mask the effects of differentexperimental conditions or genetic disease models, therefore care mustbe taken when using different protocols in different lines.

Some recently published protocols may enable the same protocol to beused for multiple lines, they also produce cardiomyocytes with very highpurity. However, pure cardiomyocytes do not facilitate the formation offunctional tissue engineered myocardium and both cardiomyocytes andstromal cells are required for the formation of functional tissueengineered myocardium (Naito et al. Circulation 114, 172-78 (2006),Hudson et al. Tissue Eng Part A 17, 2279-2289 (2011)).

Hence, there is a need in the art for methods for producingbioengineered human myocardium, which are capable of overcoming theabove disadvantages.

The development of a robust differentiation protocol is a very importantstep allowing the consistent production of BHM. In this study n>140 BHMin >18 independent experiments were produced and every one exhibitedspontaneous beating activity. Additionally, the protocol enables toproduce BHM from multiple hPSC lines using the same protocol. Inaddition, all disassociation steps could be eliminated and hPSCs weredifferentiated directly into bioengineered myocardium, thus retainingthe developmental memory of the tissue, prevent any tissue recreationresponse and provide a more accurate in vitro model of human myocardialdevelopment.

SUMMARY OF THE INVENTION

The present invention relates to a method for producing bioengineeredheart muscle from pluripotent stem cells, comprising the steps of

-   -   (i) cultivating pluripotent stem cells in a basal medium        comprising an effective amount of (a) BMP4, Activin A, FGF2, a        GSK3-inhibitor, and (b) a serum-free supplement resulting in a        final concentration of 0.5-50 mg/ml albumin, 1-100 μg/ml        transferrin, 0.1-10 μg/ml ethanol amine, 0.003-0.3 μg/ml sodium        selenite, 0.4-40 μg/ml L-Carnitine HCl, 0.1-10 μg/ml        Hydrocortisone, 0.05-5 μl/ml Fatty acid supplement, 0.0001-0.1        μg/ml triodo-L-thyronine (T3), thereby inducing mesoderm        differentiation of said pluripotent stem cells;    -   (ii) cultivating the cells obtained in step (i) in a basal        medium comprising an effective amount of an inhibitor of the        Wnt-signaling pathway and a serum-free supplement as defined in        step (i), thereby inducing cardiac differentiation of the cells;        and    -   (iii) cultivating the cells obtained in step (ii) in a basal        medium comprising an effective amount of a serum-free supplement        as defined in step (i), under mechanical stimulation, thereby        promoting cardiac maturation.

Carrying out the method disclosed herein human pluripotent stem cell(hPSC)-derived bioengineered heart muscle (BHM) is generated by directedtissue formation of hPSCs in collagen hydrogels. To form BHM, in vivodevelopment was mimicked using a directed serum-free induction protocolcausing the tissue to progress through distinct, known developmentalstages, through pluripotency, early mesoderm, cardiac progenitor,immature cardiomyocytes and finally to more mature cardiac tissuecomprised of 50% cardiomyocytes, with the rest being predominately astromal cell fraction. The inventors optimized their serum-free BHMprotocol and found that individual BHM properties are highly dependenton particular stimuli, thus indicating that multiple exogenous stimuliare required for optimal BHM properties. In the end rhythmicallycontractile BHM was produced with measurable contractile force, abilityfor pacing and inotropy in response to increased resting length, calciumconcentration and β-adrenergic stimulation. This BHM protocol, withoutmodification, was capable of consistently producing BHM from multiplehPSC lines (in every BHM in every experiment conducted).

The present data suggests that the BHM protocol disclosed herein is arobust, serum-free and reproducible way to produce human myocardium formultiple applications. For example, it is also demonstrated that BHM isa potential model of human myocardium development, and shown thatinhibition of BMP signalling leads to a more immature cardiac phenotypewith reduced contractile strength.

Accordingly, the present invention is also directed to a BHM produced bythe method according to the invention.

Further contemplated is the use of the BHM according to the invention inan in vitro-model for drug toxicity screening. In other words, thepresent invention is also directed to a method for screening drugtoxicity, comprising the step of contacting a BHM according to theinvention with a drug to be screened.

Moreover, the present invention is directed to the use of the BHMaccording to the invention in an in vitro method for testing of cardiacfunction modulation by pharmacological candidate agents. Thus, alsodescribed is a method for testing of cardiac function modulation,comprising the step of contacting a BHM according to the invention witha pharmacological candidate agent.

Finally, the present invention is also directed to the use of the BHMaccording to the invention as a research tool, as well as to a BHMaccording to the invention for use in medicine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A method for producing bioengineered heart muscle from pluripotent stemcells, comprising the steps of

-   -   (i) cultivating pluripotent stem cells in a basal medium        comprising an effective amount of (a) BMP4, Activin A, FGF2, a        GSK3-inhibitor, and (b) a serum-free supplement resulting in a        final concentration of 0.5-50 mg/ml albumin, 1-100 μg/ml        transferrin, 0.1-10 μg/ml ethanol amine, 0.003-0.3 μg/ml sodium        selenite, 0.4-40 μg/ml L-Carnitine HCl, 0.1-10 μg/ml        Hydrocortisone, 0.05-5 μl/ml Fatty acid supplement, 0.0001-0.1        μg/ml triodo-L-thyronine (T3), thereby inducing mesoderm        differentiation of said pluripotent stem cells;    -   (ii) cultivating the cells obtained in step (i) in a basal        medium comprising an effective amount of an inhibitor of the        Wnt-signaling pathway and a serum-free supplement as defined in        step (i), thereby inducing cardiac differentiation of the cells;        and    -   (iii) cultivating the cells obtained in step (ii) in a basal        medium comprising an effective amount of a serum-free supplement        as in (i), under mechanical stimulation, thereby promoting        cardiac maturation.

In a preferred embodiment, the pluripotent stem cells are pluripotentstem cells of primate origin, more preferably the pluripotent stem cellsare human pluripotent stem cells. Pluripotent stem cells are able todifferentiate into every cell type of the body. As such, humanpluripotent stem cells offer the unique opportunity to obtain bona fidehuman heart cells. Currently, the most utilized pluripotent cells areembryonic stem cells (ESC) or induced pluripotent stem cells (iPSC).Human ESC-lines were first established by Thomson and coworkers (Thomsonet al., Science 282: 1145-1147 (1998); incorporated herein in itsentirety by reference). Human ESC research recently enabled thedevelopment of a new technology to reprogram cells of the body into aES-like cell. This technology was pioneered by Yamanaka and coworkers in2006 (Takahashi & Yamanaka Cell 126: 663-676 (2006); incorporated hereinin its entirety by reference). Resulting induced pluripotent cells(iPSC) show a very similar behavior as ESC and, importantly, are alsoable to differentiate into every cell of the body. Moreover, it wasreported that also parthenogenetic stem cells are likely to be suitablefor BHM-production (Didié et al. J Clin Invest. 123, 1285-1298 (2013);incorporated herein in its entirety by reference). Accordingly, thepluripotent stem cells can be selected from embryonic stem cells,induced pluripotent stem cells, and parthenogenetic stem cells. In thecontext of the present invention, said pluripotent stem cells arehowever not produced using a process which involves modifying the germline genetic identity of human beings or which involves use of a humanembryo for industrial or commercial purposes.

The basal medium used in step (i), can be selected from DMEM/F12,StemPro, Iscove's medium, αMEM, DMEM, and RPMI. Preferably, the basalmedium used in step (i) is RPMI supplemented with pyruvate. However anysuitable basal medium may be used in the method. Basal mediums arecommercially available or may be prepared according to recipes which arepublicly available, e.g. from catalogues of the ATCC. If deemedappropriate, the basal medium may be supplemented with non-essentialamino acids. If αMEM is used as the basal medium, the basal medium neednot be supplemented additionally with non-essential amino acids. Thenon-essential amino acids are commercially available as a combinedsupplement. Such a supplement for example comprises 750 mg/L glycine,890 mg/L L-alanine, 1320 mg/L L-asparagine, 1330 mg/L L-aspartic acid,1470 mg/L L-glutamic acid, 1150 mg/L L-proline, and 1050 mg/L L-serine.

As set out above, the basal medium of step (i) comprises an effectiveamount of BMP4, Activin A, FGF2, and a GSK3-inhibitor. For example, sucha basal medium comprises 1-20 ng/ml BMP4, preferably 2-15 ng/ml, morepreferably 2.5-10 ng/ml, more preferably 3-8 ng/ml, most preferably 4-6ng/ml, and even most preferably about 5 ng/ml;

0.1-10 ng/ml FGF2, preferably 1-9 ng/ml, more preferably 2-8 ng/ml, evenmore preferably 3-7 ng/ml, most preferably 4-6 ng/ml, and even mostpreferably about 5 ng/ml,1-20 ng/ml Activin A, preferably 2.5-18 ng/ml, more preferably 5-16ng/ml, even more preferably 7.5-14 ng/ml, still more preferably 8-12ng/ml, most preferably 8.5-10 ng/ml, and even most preferably about 9ng/ml.

The GSK3-inhibitor in the basal medium of step (i) can be selected, forexample, from the group consisting of CHIR99021, CHIR98014, SB216763,TWS119, Tide-glusib, SB415286, and LY2090314. However, anyGSK3-inhibitor suitable in the method of the invention can be applied.In a preferred embodiment, the GSK3-inhibitor in the basal medium ofstep (i) is CHIR99021.

It will be understood by the skilled person that the concentration of aneffective amount of a GSK3-inhibitor varies with the availability andinhibition constant of the inhibitor in question. In the context of thepresent invention, the term “effective amount” as used herein in thecontext of a GSK3-inhibitor is intended to mean an enzyme inactivatingconcentration. For example, in case of CHIR99021, the basal medium instep (i) comprises 0.1-10 μM CHIR99021, preferably 0.2-9 μM, morepreferably 0.3-8 μM, even more preferably 0.4-7 μM, still morepreferably 0.5-6 μM, more preferably 0.6-5 μM, more preferably 0.7-4 μM,more preferably 0.8-3 μM, most preferably 0.9-2 μM, and even mostpreferably about 1 μM CHIR99021. It will be understood that an effectiveconcentration of any receptor/enzyme agonist or inhibitor varies withthe availability and biological activity of the respective compound. Theserum-free supplement applied in step (i), (ii) and (iii) of the methodis formulated to result in a final concentration of 0.5-50 mg/ml albumin(preferably 1-40 mg/ml, more preferably 2-30 mg/ml, still morepreferably 3-20 mg/ml, most preferably 4-10 mg/ml, and even mostpreferably 4.5-7.5 mg/ml such as about 5 mg/ml), 1-100 μg/ml transferrin(preferably 2-90 μg/ml, more preferably 3-80 μg/ml, even more preferably4-70 μg/ml, still more preferably 5-60 μg/ml, more preferably 6-50μg/ml, more preferably 7-40 μg/ml, more preferably 8-30 μg/ml, morepreferably 9-20 μg/ml, such as about 10 μg/ml),

0.1-10 μg/ml ethanol amine (preferably 0.2-9 μg/ml, more preferably0.3-8 μg/ml, even more preferably 0.4-7 μg/ml, still more preferably0.5-6 μg/ml, more preferably 0.6-5 μg/ml, more preferably 0.7-4 μg/ml,more preferably 0.8-3 μg/ml, more preferably 1-2.5 μg/ml, such as about2 μg/ml),0.003-0.3 μg/ml sodium selenite (preferably 0.005-0.2 μg/ml, morepreferably 0.01-0.1 μg/ml, even more preferably 0.02-0.05 μg/ml, andmost preferably about 0.03 μg/ml, such as about 0.032 μg/ml),0.4-40 μg/ml L-Carnitine HCl (preferably 0.5-30 μg/ml, more preferably1-20 μg/ml, even more preferably 2-10 μg/ml, most preferably 3-5 μg/ml,and even most preferably about 4 μg/ml),0.1-10 μg/ml Hydrocortisone (preferably 0.2-9 μg/ml, more preferably0.3-8 μg/ml, even more preferably 0.4-7 μg/ml, still more preferably0.5-6 μg/ml, more preferably 0.6-5 μg/ml, more preferably 0.7-4 μg/ml,more preferably 0.8-3 μg/ml, more preferably 0.9-2 μg/ml, such as about1 μg/ml),0.05-5 μl/ml Fatty acid supplement (preferably 0.1-4 μl/ml, morepreferably 0.2-3 μl/ml, even more preferably 0.3-3 μl/ml, mostpreferably 0.4-2 μl/ml, and even most preferably 0.45-1 μl/ml, such asabout 0.5 μl/ml), and0.0001-0.1 μg/ml triodo-L-thyronine (T3) (preferably 0.001-0.01 μg/ml,more preferably 0.002-0.0075 μg/ml, even more preferably 0.003-0.005μg/ml, and most preferably about 0.004 μg/ml).

In addition, the serum-free supplement may further comprise one or morecomponents selected from the group consisting of vitamin A, D-galactose,L-carnitine, linoleic acid, linolenic acid, progesterone, andputrescine. These components are conducive for the viability of thecells. Suitable concentrations of the respective components are known tothe skilled person or can be easily determined using routine measures.

The serum-free supplement referred to in step (i) is also commerciallyavailable. For example, B27® supplement or B27® supplement minus insulincan be used. In a preferred embodiment, the B27® supplement or B27®supplement minus insulin used in step (i) of the above method is appliedin an amount of 0.1-10% B27® or B27® minus insulin, preferably 0.5-8%,more preferably 1-6%, even more preferably 1.5-4% , and most preferablyabout 2% B27® or B27® minus insulin.

As demonstrated in the examples below, it has been proven advantageousto include an effective amount of ascorbic acid or a salt or aderivative thereof into the basal medium of step (i). In a preferredembodiment, the basal medium of step (i) comprises 10-1000 μM,preferably 50-400 μM, more preferably 100-300 μM, even more preferably150-250 μM, and most preferably about 200 μM of ascorbic acid or a saltor a derivative thereof. The ascorbic acid may be delivered in the freeform or as a salt. Since ascorbate is the active ingredient, any salt orderivative of ascorbic acid may be used, which provides the ascorbate tothe cells, provided the counter ion has no detrimental effect on thecells. As shown in the examples, one suitable salt or derivative ofascorbic acid is ascorbate-2-phosphate.

The length of step (i) and the concentration of factors such as BMP4,Activin A, FGF2, and the GSK3-inhibitor may be optimized by monitoringthe efficiency of induction of mesoderm differentiation. This can beachieved by monitoring the expression of cell surface or pluripotencymarkers, i.e. by (a) a decrease of TRA-1-60 and OCT4 positive cells(pluripotent stem cells) and (b) an increase of MIXL1 and Mesp1 positivecells (mesoderm) (see also FIG. 4f herein).

Briefly, cells are fixed using ethanol, blocked using standardprotocols, and then stained with primary antibodies directed againstTRA-1-60, OCT4, MIXL1 and/or Mesp1 (cf. Table 2 below) in blockingbuffer for 45 min, optionally followed by secondary antibodies (if theprimary antibody is not fluorescence labelled) in blocking buffer andHoechst for 30 min at 4° C. (cf. Table 2 below). A BD LSRII is used forflow cytometry analysis (BD Biosystems). For live cells populations aregated based on forward-side scatter profiles. BD FACSDiva Software (BDBioscience) or Cyflologic v1.2.1 (Cyflo Ltd) are used for analysis.Induction of mesoderm differentiation is indicated if

(a) less than 50%, preferably less than 40%, more preferably less than30%, even more preferably less than 20%, most preferably less than 10%,and even most preferably less than 5% of the cells of the live cellspopulation are positive for TRA-1-60; and/or less than 50%, preferablyless than 40%, more preferably less than 30%, even more preferably lessthan 20%, most preferably less than 10%, and even most preferably lessthan 5% of the cells of the live cells population are positive for OCT4;and(b) more than 20%, preferably more than 30%, more preferably more than40%, even more preferably more than 50%, and most preferably more than60%, of the cells of the live cells population are positiv for MIXL1;and/or more than 20%, preferably more than 30%, more preferably morethan 40%, even more preferably more than 50%, and most preferably morethan 60% of the cells of the live cells population are positive forMesp1.

Usually, step (i) is carried out for 48-96 h. Preferably, step (i) iscarried out for 60-84 h, and more preferably step (i) is carried out for66-78 h.

The basal medium used in step (ii), can be selected from DMEM/F12,StemPro, Iscove's medium, αMEM, DMEM, and RPMI. Preferably, the basalmedium used in step (ii) is RPMI supplemented with pyruvate. However anysuitable basal medium may be used in the method.

If deemed appropriate, the basal medium of step (ii) may be supplementedwith non-essential amino acids. If αMEM is used as the basal medium instep (ii), the basal medium need not be supplemented additionally withnon-essential amino acids. The non-essential amino acids arecommercially available as a combined supplement. Such a supplement forexample comprises 750 mg/L glycine, 890 mg/L L-alanine, 1320 mg/LL-asparagine, 1330 mg/L L-aspartic acid, 1470 mg/L L-glutamic acid, 1150mg/L L-proline, and 1050 mg/L L-serine.

The basal medium in step (ii) may be independently selected from thebasal medium applied in step (i). However, in a preferred embodiment,the basal medium in steps (i) and (ii) is the same.

The inhibitor of the Wnt-signaling pathway in the basal medium of step(ii) may be any inhibitor of the Wnt-signaling pathway, which can besuitably applied in the method of the invention. Preferably, saidinhibitor of the Wnt-signaling pathway is selected from the groupconsisting of IWP4, IWP2, IWR-1, IWP1, IWP3, IWR-2, IWR-3, IWR-4, IWR-5,XAV939, DKK1, quercetin, ICG-001, pyrvinium, CCT031374, iCRT-3,5,14,CPG049090, and NC043. More preferably said inhibitor of theWnt-signaling pathway is selected from the group consisting of IWP4,IWP2, IWR-1, IWP1, IWP3, IWR-2, IWR-3, IWR-4, IWR-5, XAV939, DKK1. Asdemonstrated in the examples below, one particularly useful inhibitor ofthe Wnt-signaling pathway in the basal medium of step (ii) is IWP4.

The serum-free supplement referred to in step (ii) is as defined forstep (i) above. The serum-free supplements applied in step (i) and (ii)may be the same or not. Likewise, B27® supplement or B27® supplementminus insulin can be used in step (ii). In a preferred embodiment, theB27® supplement or B27® supplement minus insulin used in step (ii) ofthe above method is applied in an amount of 0.1-10% B27® or B27® minusinsulin, preferably 0.5-8%, more preferably 1-6%, even more preferably1.5-4% , and most preferably about 2% B27® or B27® minus insulin.

It will be understood by the skilled person that the concentration of aneffective amount of an inhibitor of the Wnt-signaling pathway varieswith the availability and inhibition constant of the inhibitor inquestion. For example, in case of IWP4, the basal medium of step (ii)may comprise 0.1-10 μM IWP4, preferably 1-9 μM, more preferably 2-8 μM,even more preferably 3-7 μM, still more preferably 4-6 μM, and mostpreferably about 5 μM IWP4. It will be understood that an effectiveconcentration of any receptor/enzyme agonist or inhibitor varies withthe availability and biological activity of the respective compound.

As demonstrated in the examples below, it has been proven advantageousto include an effective amount of ascorbic acid or a salt or aderivative thereof into the basal medium of step (ii). In a preferredembodiment, the basal medium of step (ii) comprises 10-1000 μM,preferably 50-400 μM, more preferably 100-300 μM, even more preferably150-250 μM, and most preferably about 200 μM of ascorbic acid or a saltor a derivative thereof. The ascorbic acid may be delivered in the freeform or as a salt. Since ascorbate is the active ingredient, any salt orderivative of ascorbic acid may be used, which provides the ascorbate tothe cells, provided the counter ion has no detrimental effect on thecells. As shown in the examples, one suitable salt or derivative ofascorbic acid for use in the basal medium in step (ii) isascorbate-2-phosphate.

The length of step (ii) and the concentration of the remainingconstituents such as the inhibitor of the Wnt-signaling pathway may beoptimized by monitoring the efficiency of induction of cardiacdifferentiation of the cells. This can be achieved by monitoring theexpression of differentiation markers, i.e. by an increase of Nkx2.5 andactinin.

Briefly, cells are fixed using ethanol, blocked, and then stained withprimary antibodies directed against Nkx2.5 and/or actinin (cf. Table 2below) in blocking buffer for 45 min, optionally followed by secondaryantibodies (if the primary antibody is not fluorescence labelled) inblocking buffer and Hoechst for 30 min at 4° C. (cf. Table 2 below). ABD LSRII is used for flow cytometry analysis (BD Biosystems). For livecells populations are gated based on forward-side scatter profiles. BDFACSDiva Software (BD Bioscience) or Cyflologic v1.2.1 (Cyflo Ltd) areused for analysis. Induction of cardiac differentiation is indicated ifmore than 20%, preferably more than 30%, more preferably more than 40%,even more preferably more than 50%, and most preferably more than 60%,of the cells of the live cells population are positiv for Nkx2.5; and/ormore than 20%, preferably more than 30%, more preferably more than 40%,even more preferably more than 50%, and most preferably more than 60% ofthe cells of the live cells population are positive for actinin (seealso FIG. 4d and 4f herein).

Usually, step (ii) is carried out for 8-12 days. Preferably, step (ii)is carried out for 9-11 days, and most preferably step (ii) is carriedout for 10 days.

The basal medium used in step (iii), can be selected from DMEM/F12,StemPro, Is-cove's medium, αMEM, DMEM, and RPMI. Preferably, the basalmedium used in step (iii) is RPMI supplemented with pyruvate. Howeverany suitable basal medium may be used in the method.

If deemed appropriate, the basal medium of step (iii) may besupplemented with non-essential amino acids. If aMEM is used as thebasal medium in step (iii), the basal medium need not be supplementedadditionally with non-essential amino acids. The non-essential aminoacids are commercially available as a combined supplement. Such asupplement for example comprises 750 mg/L glycine, 890 mg/L L-alanine,1320 mg/L L-asparagine, 1330 mg/L L-aspartic acid, 1470 mg/L L-glutamicacid, 1150 mg/L L-proline, and 1050 mg/L L-serine.

The basal medium in step (iii) may be independently selected from thebasal medium applied in steps (i) and/or (ii). However, in a preferredembodiment, the basal medium in steps (ii) and (iii) is the same. Morepreferably, the basal medium in steps (i), (ii) and (iii) is the same.

As demonstrated in the examples below, it has been proven advantageousto include an effective amount of ascorbic acid or a salt or aderivative thereof into the basal medium of step (iii). In a preferredembodiment, the basal medium of step (iii) comprises 10-1000 μM,preferably 50-400 μM, more preferably 100-300 μM, even more preferably150-250 μM, and most preferably about 200 μM of ascorbic acid or a saltor a derivative thereof. The ascorbic acid may be delivered in the freeform or as a salt. Since ascorbate is the active ingredient, any salt orderivative of ascorbic acid may be used, which provides the ascorbate tothe cells, provided the counter ion has no detrimental effect on thecells. As shown in the examples, one suitable salt or derivative ofascorbic acid for use in the basal medium in step (iii) isascorbate-2-phosphate.

The serum-free supplement referred to in step (iii) is a serum-freesupplement as defined for step (i) above. The serum-free supplementsapplied in steps (i), (ii) and (iii) may be the same or not. Likewise,B27® supplement or B27® supplement minus insulin can be used in step(iii). In a preferred embodiment, the B27® supplement or B27® supplementminus insulin used in step (iii) of the above method is applied in anamount of 0.1-10% B27® or B27® minus insulin, preferably 0.5-8%, morepreferably 1-6%, even more preferably 1.5-4% , and most preferably about2% B27® or B27® minus insulin.

The basal medium of step (iii) further comprises an effective amount ofTGFβ1. For example, the basal medium of step (iii) may comprise 0.1-10ng/ml TGFβ1, preferably 0.2-9 ng/ml, more preferably 0.3-8 ng/ml, evenmore preferably 0.4-7 ng/ml, still more preferably 0.5-6 ng/ml, morepreferably 0.6-5 ng/ml, more preferably 0.7-4 ng/ml, more preferably0.8-3 ng/ml, most preferably 0.9-2 ng/ml, and even most preferably about1 ng/ml TGFβ1.

As shown in the examples, it is advantageous for cardiac maturation ifthe basal medium of step (iii) does not comprise an effective amount ofFGF2. In contrast thereto, calcium has been shown to improve cardiacmaturation. Accordingly, in a preferred embodiment, the basal medium ofstep (iii) comprises 0.5-3 mM Ca²⁺, preferably 0.5-2.75 mM Ca²⁺, morepreferably 1-2.25 mM Ca²⁺, even more preferably 1-1.5 mM mM Ca²⁺, andmost preferably about 1.2 mM Ca²⁺.

Usually, step (iii) of the method of the invention is carried out undermechanical stimulation, e.g. on a stretch device, as generally known inthe art. Preferably, the stretch device applies a static, phasic ordynamic stretch to the BHM. More specifically, mechanical stretching canbe (a) static, (b) dynamic, or (c) flexible against a resilient load.Preferably, the mechanical stimulation in step (iii) is dynamicmechanical stimulation or static stretch. In a more preferredembodiment, the mechanical stimulation in step (iii) is dynamicmechanical stimulation against a resilient load to facilitate auxotoniccontractions.

Whether cardiac maturation is promoted can be tested by opticalinspection for spontaneous or electrically stimulated contractions.Preferably, cardiac maturation is monitored by an isometric contractionexperiment, wherein a twitch force development of >0.01 mN is indicativefor cardiac maturation.

Briefly, contraction experiments are performed in organ baths at 37° C.under constant bubbling with 5% CO₂ and 95% O₂ to maintain aphysiological pH in Tyrode's solution containing (all in mM): 120 NaCl,1 MgCl₂, 0.2 CaCl₂, 5.4 KCl, 22.6 NaHCO₃, 4.2 NaH₂PO₄, 5.6 glucose and0.56 ascorbate. Calcium is adjusted using a 0.2 M calcium chloridesolution. All BHM are analysed at 3 Hz with 5 ms square pulses of 200 mAelectrical current in order to pace at approximately the embryonic heartrate. Stimulation frequency is altered to confirm proper force-frequencyresponse (Bowditch mechanism). BHM are mechanically stretched atintervals of 125 μm until the maximum twitch force is observed(force-length response; Frank-Starling mechanism).

Usually, step (iii) is carried out for at least 72 h. Although there isno particular upper limit for the length of step (iii), said step isusually carried out for less than 100 days. In specific embodiments,step (iii) may be carried out for 4-50 days, such as for about 15 days.

Step (i) of the method of the invention may be preceded by a seedingstep, wherein said pluripotent stem cells are seeded in a ratio of(2.5-6×10⁶ cells/1 mg collagen)/1 ml medium in a suitable mould.Preferably, the seeding step is carried out 18-30 h prior to step (i).

The medium used in the seeding step usually comprises 0.2-2 mg/mlcollagen (preferably 0.3-1.9 mg/ml, more preferably 0.4-1.8 mg/ml, evenmore preferably 0.4-1.7 mg/ml, still more preferably 0.5-1.6 mg/ml, morepreferably 0.6-1.5 mg/ml, more preferably 0.7-1.4 mg/ml, more preferably0.8-1.3 mg/ml, more preferably 0.9-1.2 mg/ml, such as about 1 mg/ml).The collagen is preferably of medical grade and selected from the groupconsisting of collagen type I, collagen type III, collagen type V, and amixture thereof. In a more preferred embodiment, at least 90% of saidcollagen is collagen type I. However, said collagen may also furthercomprises one or more extracellular matrix components selected from thegroup consisting of elastin, laminin, entactin, nidogen, proteoglycan,and fibronectin. Usually, the exact composition of the collagen willdepend on the origin, from where it is derived from. The collagen ispreferably of human origin, but bovine or porcine origin, or marineorigin, such as from algae or fish origin, is also contemplated.Alternatively, recombinant collagen may also be used.

In order to achieve suitable cell densities, for some pluripotent celllines it may be helpful to supplement the medium used in the seedingstep with a ROCK-inhibitor. Therefore, in a preferred embodiment, themedium used in the seeding step further comprises a ROCK-inhibitor. TheROCK-inhibitor may be any ROCK-inhibitor, which can be suitably appliedin the method of the invention. Preferably, said ROCK inhibitor isselected from Y27632, H-1152P, Thiazovivin, Fasudil, Hydroxyfasudil,GSK429286A, and RKI-1447, preferably selected from Y27632, H-1152P,Thiazovivin, Fasudil, Hydroxyfasudil, and more preferably the ROCKinhibitor is selected from Y27632 or H-1152P. As demonstrated in theexamples below, one particularly useful ROCK-inhibitor is Y27632.

It will be understood by the skilled person that the concentration of aneffective amount of a ROCK-inhibitor varies with the availability andinhibition constant of the inhibitor in question. For example, in caseof Y27632, the medium used in the seeding step may comprise 1-50 μM,preferably 2.5-40 μM, more preferably 5-30 μM, even more preferably7.5-20 μM, most preferably 8-12 μM, and most preferably about 10 μMY27632.

It will be understood that an effective concentration of anyreceptor/enzyme agonist or inhibitor varies with the availability andbiological activity of the respective compound.

Apart from the above disclosed method, the invention further relates toa BHM produced by said method. Despite the increased maturity observedin our BHM protocol, it should also be noted that the BHM is still arelatively immature tissue. Compared to adult heart tissue the BHM stillhas an inferior β-MHC/α-MHC ratio, and low but still retained expressionof progenitor genes (e.g. ISL1). However, prolonged culture underappropriate culture conditions with biophysical stimulation may furtherincrease maturity. There is already morphological evidence suggestingthat this may also be the case in the BHM system.

The BHM obtained by the method disclosed herein exhibits the followingcharacteristics: It can be paced at multiple frequencies up to at least3 Hz, exhibits a calcium EC₅₀ of higher than 0.2 mM being preferably inthe physiological range 4-8 mM, and a twitch tension of more than 200μN. The twitch tension is increased in response to increased restinglength and resting tension. In response to 1 μM isoprenaline, the BHMexhibits an inotropic response of more than 40 μN under paced conditionsat 0.6 mM calcium, preferably more than 45 μN, more preferably more than50 μN.

Briefly, all contraction experiments are performed in organ baths at 37°C. and physiological pH in Tyrode's solution containing (all in mM): 120NaCl, 1 MgCl₂, 0.2 CaCl₂, 5.4 KCl, 22.6 NaHCO₃, 4.2 NaH₂PO₄, 5.6 glucoseand 0.56 ascorbate. Calcium is adjusted using a 0.2 M calcium chloridesolution. All BHM are analysed at 3 Hz with 5 ms square pulses of 200 mAelectrical current in order to pace at approximately the embryonic heartrate. Stimulation frequency is altered to confirm proper force-frequencyresponse (Bowditch mechanism). BHM are mechanically stretched atintervals of 125 μm until the maximum twitch force is observed at 2 mMcalcium (Frank-Starling mechanism). Subsequently, BHM are subjected todifferent calcium concentrations (0.2, 0.4, 0.8, 1.2, 1.6, 2.0, 2.4 mM)and the twitch force is recorded. For isoprenaline experiments thecalcium concentration is adjusted to 0.6 mM and subsequently theisoprenaline concentration is adjusted to 1 μM.

Another characteristic of the BHM obtained by the method disclosedherein is that it comprises CD90⁺ stromal cells. Expression of CD90 canbe determined using flow cytometry. Briefly, cells are fixed usingethanol, blocked, and then stained with primary antibodies directedagainst CD90 (cf. Table 2 below) in blocking buffer for 45 min,optionally followed by secondary antibodies (if the primary antibody isnot fluorescence labelled) in blocking buffer and Hoechst for 30 min at4° C. (cf. Table 2 below). A BD LSRII is used for flow cytometryanalysis (BD Biosystems). For live cells populations are gated based onforward-side scatter profiles. BD FACSDiva Software (BD Bioscience) orCyflologic v1.2.1 (Cyflo Ltd) are used for analysis.

The BHM may provide a good model system for investigating mechanismsdriving maturation in a serum-free environment, and we have alreadydemonstrated that with increased culture periods maturity may beincreased (we showed increase isoprenaline sensitivity and improvetissue morphology). The capability of long term BHM culture without lossof function (at least 63 days) also suggests that long termpharmacological safety and efficacy experiments are possible. Hence, ina preferred embodiment, the BHM obtained by the method disclosed hereincan be maintained for at least 63 days.

Using the traditional approach of differentiation followed by tissueengineering the differentiation cultures require extensive digestionprotocols in order to yield single cell/small clumps required forcardiac tissue engineering applications. These digestion protocolsdestroy the extracellular environment and spatial distribution formedduring development and may hence have a difficult to control inhibitoryeffect on the cardiac differentiation protocol.

Using the BHM as a model we demonstrated that factors affecting earlydevelopment (ASC-2-P, dorsomorphin) and later development (mechanicalstimulation, FGF2, TGFβ1, and calcium concentration) had a profoundimpact on BHM function and properties. Therefore, our BHM protocol maybe a useful tool in the study of developmental processes not onlygoverning cardiogenesis, but also tissue formation and properties.

Accordingly, the BHM obtained by the method disclosed herein can besuitably used as a research tool. For example, the use of the BHMobtained by the method disclosed herein in an in vitro-model for drugtoxicity screening is contemplated. In other words, a method forscreening drug toxicity is contemplated, comprising the step ofcontacting a BHM obtained by the method disclosed herein with a drug tobe screened. Alternatively, the BHM obtained by the method disclosedherein may be used in an in vitro method for testing of cardiac functionmodulation by pharmacological candidate agents. Thus, also described isa method for testing of cardiac function modulation, comprising the stepof contacting a BHM according to the invention with a pharmacologicalcandidate agent.

Finally, the BHM obtained by the method disclosed herein can be used inmedicine. Merely as an example, it is contemplated that the BHM obtainedby the method disclosed herein can be advantageously used in heartrepair.

The invention is further described by the following embodiments:

-   -   1. A method for producing bioengineered heart muscle from        pluripotent stem cells, comprising the steps of    -   (i) cultivating pluripotent stem cells in a basal medium        comprising an effective amount of (a) BMP4, Activin A, FGF2, a        GSK3-inhibitor, and (b) a serum-free supplement resulting in a        final concentration of 0.5-50 mg/ml albumin, 1-100 μg/ml        transferrin, 0.1-10 μg/ml ethanol amine, 0.003-0.3 μg/ml sodium        selenite, 0.4-40 μg/ml L-Carnitine HCl, 0.1-10 μg/ml        Hydrocortisone, 0.05-5 μl/ml Fatty acid supplement, 0.0001-0.1        μg/ml triodo-L-thyronine (T3), thereby inducing mesoderm        differentiation of said pluripotent stem cells;    -   (ii) cultivating the cells obtained in step (i) in a basal        medium comprising an effective amount of an inhibitor of the        Wnt-signaling pathway and a serum-free supplement as in (i),        thereby inducing cardiac differentiation of the cells; and    -   (iii) cultivating the cells obtained in step (ii) in a basal        medium comprising an effective amount of a serum-free supplement        as in (i), under mechanical stimulation, thereby promoting        cardiac maturation.

2. The method of embodiment 1, wherein the pluripotent stem cells arepluripotent stem cells of primate origin, preferably human pluripotentstem cells.

3. The method of embodiment 1 or 2, wherein the pluripotent stem cellsare selected from embryonic stem cells, induced pluripotent stem cells,and parthenogenetic stem cells.

4. The method of any one of embodiment 1-3, wherein the basal medium, ofstep (i) comprises 10-1000 μM, preferably 50-400 μM, more preferably100-300 μM, even more preferably 150-250 μM, and most preferably about200 μM of ascorbic acid or a salt or a derivative thereof.

5. The method of embodiment 4, wherein the salt or derivative ofascorbic acid is ascorbate-2-phosphate.

6. The method of any one of embodiment 1-5, wherein step (i) is carriedout for 48-96 h, preferably wherein step (i) is carried out for 60-84 h,most preferably wherein step (i) is carried out for 66-78 h.

7. The method of any one of embodiment 1-6, wherein the basal medium instep (i) comprises 1-20 ng/ml BMP4, preferably 2-15 ng/ml, morepreferably 2.5-10 ng/ml, more preferably 3-8 ng/ml, most preferably 4-6ng/ml, and even most preferably about 5 ng/ml.

8. The method of any one of embodiment 1-7, wherein the basal medium instep (i) comprises 0.1-10 ng/ml FGF2, preferably 1-9 ng/ml, morepreferably 2-8 ng/ml, even more preferably 3-7 ng/ml, most preferably4-6 ng/ml, and even most preferably about 5 ng/ml.

9. The method of any one of embodiment 1-8, wherein the basal medium instep (i) comprises 1-20 ng/ml Activin A, preferably 2.5-18 ng/ml, morepreferably 5-16 ng/ml, even more preferably 7.5-14 ng/ml, still morepreferably 8-12 ng/ml, most preferably 8.5-10 ng/ml, and even mostpreferably about 9 ng/ml.

10. The method of any one of embodiment 1-9, wherein the GSK3-inhibitorin the basal medium of step (i) is selected from the group consisting ofCHIR99021, CHIR98014, SB216763, TWS119, Tideglusib, SB415286, andLY2090314.

11. The method of embodiment 10, wherein the GSK3-inhibitor in the basalmedium of step (i) is CHIR99021.

12. The method of embodiment 11, wherein the basal medium in step (i)comprises 0.1-10 μM CHIR99021, preferably 0.2-9 μM, more preferably0.3-8 μM, even more preferably 0.4-7 μM, still more preferably 0.5-6 μM,more preferably 0.6-5 μM, more preferably 0.7-4 μM, more preferably0.8-3 μM, most preferably 0.9-2 μM, and even most preferably about 1 μMCHIR99021.

13. The method of any one of embodiment 1-12, wherein the serum-freesupplement in step (i) comprises 0.1-10% B27 or B27 minus insulin,preferably 0.5-8%, more preferably 1-6%, even more preferably 1.5-4% ,and most preferably about 2% B27 or B27 minus insulin.

14. The method of any one of embodiment 1-13, wherein the basal mediumused in step (i), is DMEM/F12, StemPro, Iscove's medium, αMEM, DMEM, andRPMI.

15. The method of claim 14, wherein the basal medium used in step (i) isRPMI supplemented with pyruvate.

16. The method of any one of embodiment 1-15, wherein the inhibitor ofthe Wnt-signaling pathway in the basal medium of step (ii) is selectedfrom the group consisting of IWP4, IWP2, IWR-1, IWP1, IWP3, IWR-2,IWR-3, IWR-4, IWR-5, XAV939, DKK1, quercetin, ICG-001, pyrvinium,CCT031374, iCRT-3,5,14, CPG049090, NC043; preferably selected from thegroup consisting of IWP4, IWP2, IWR-1, IWP1, IWP3, IWR-2, IWR-3, IWR-4,IWR-5, XAV939, DKK1.

17. The method of embodiment 16, wherein the inhibitor of theWnt-signaling pathway in the basal medium of step (ii) is IWP4.

18. The method of embodiment 17, wherein the basal medium of step (ii)comprises 0.1-10 μM IWP4, preferably 1-9 μM, more preferably 2-8 μM,even more preferably 3-7 μM, still more preferably 4-6 μM, and mostpreferably about 5 μM IWP4.

19. The method of any one of embodiment 1-18, wherein the basal mediumof step (ii) comprises 10-1000 μM, preferably 50-400 μM, more preferably100-300 μM, even more preferably 150-250 μM, and most preferably about200 μM of ascorbic acid or a salt or a derivative thereof.

20. The method of embodiment 19, wherein the salt or derivative ofascorbic acid is ascorbate-2-phosphate.

21. The method of any one of embodiment 1-20, wherein step (ii) iscarried out for 8-12 days, preferably wherein step (ii) is carried outfor 9-11 days, most preferably wherein step (ii) is carried out for 10days.

22. The method of any one of embodiment 1-21, wherein the serum-freesupplement in step (ii) comprises 0.1-10% B27 or B27 minus insulin,preferably 0.5-8%, more preferably 1-6%, even more preferably 1.5-4%,and most preferably about 2% B27 or B27 minus insulin.

23. The method of any one of embodiment 1-22, wherein the basal mediumused in step (ii), is DMEM/F12, StemPro, Iscove's medium, αMEM, DMEM,and RPMI.

24. The method of embodiment 23, wherein the basal medium used in step(ii) is RPMI supplemented with pyruvate.

25. The method of any one of embodiment 1-24, wherein the basal mediumof step (iii) further comprises 10-1000 μM, preferably 50-400 μM, morepreferably 100-300 μM, even more preferably 150-250 μM, and mostpreferably about 200 μM of ascorbic acid or a salt or a derivativethereof.

26. The method of embodiment 25, wherein the salt or derivative ofascorbic acid is ascorbate-2-phosphate.

27. The method of any one of embodiment 1-26, wherein step (iii) iscarried out for at least 72 h, preferably wherein step (iii) is carriedout for less than 100 days, more preferably wherein step (iii) iscarried out for 4-50 days, and most preferably wherein step (iii) iscarried out for about 15 days.

28. The method of any one of embodiment 1-27, wherein the serum-freesupplement in step (iii) comprises 0.1-10% B27 or B27 minus insulin,preferably 0.5-8%, more preferably 1-6%7, even more preferably 1.5-4%,and most preferably about 2% B27 or B27 minus insulin.

29. The method of any one of embodiment 1-28, wherein the basal mediumused in step (iii), is DMEM/F12, StemPro, Iscove's medium, αMEM, DMEM,and RPMI.

30. The method of embodiment 29, wherein the basal medium used in step(i) is RPMI supplemented with pyruvate.

31. The method of any one of embodiment 1-30, wherein the basal mediumof step (iii) further comprises 0.1-10 ng/ml TGFβ1, preferably 0.2-9ng/ml, more preferably 0.3-8 ng/ml, even more preferably 0.4-7 ng/ml,still more preferably 0.5-6 ng/ml, more preferably 0.6-5 ng/ml, morepreferably 0.7-4 ng/ml, more preferably 0.8-3 ng/ml, most preferably0.9-2 ng/ml, and even most preferably about 1 ng/ml TGFβ1.

32. The method of any one of embodiment 1-31, wherein the basal mediumof step (iii) does not comprise an effective amount of FGF2.

33. The method of any one of embodiment 1-32, wherein the basal mediumof step (iii) comprises 0.5-3 mM Ca²⁺, preferably 0.5-2.75 mM Ca²⁺, morepreferably 1-2.25 mM Ca²⁺, even more preferably 1-1.5 mM mM Ca²⁺, andmost preferably about 1.2 mM Ca²⁺.

34. The method of any one of embodiment 1-33, wherein the mechanicalstimulation in step (iii) is dynamic mechanical stimulation or staticstretch.

35. The method of embodiment 34, wherein the mechanical stimulation instep (iii) is dynamic mechanical stimulation.

36. The method of any one of embodiment 1-35, comprising prior to step(i) a seeding step, wherein said pluripotent stem cells are seeded in aratio of (2.5-6×10⁶ cells/1 mg collagen)/1 ml medium in a suitablemould.

37. The method of embodiment 36, wherein the collagen is collagen I.

38. The method of any one of embodiment 36-37, wherein said collagen isof human origin, bovine origin, or marine origin.

39. The method of any one of embodiment 36-38, wherein the medium usedin the seeding step further comprises a ROCK-inhibitor.

40. The method of embodiment 39, wherein the ROCK inhibitor is selectedfrom Y27632, H-1152P, Thiazovivin, Fasudil, Hydroxyfasudil, GSK429286A,and RKI-1447, preferably selected from Y27632, H-1152P, Thiazovivin,Fasudil, Hydroxyfasudil, and more preferably the ROCK inhibitor isY27632 or H-1152P, and most preferably the ROCK inhibitor is Y27632.

41. The method of embodiment 40, wherein the medium used in the seedingstep comprises 1-50 μM, preferably 2.5-40 μM, more preferably 5-30 μM,even more preferably 7.5-20 μM, most preferably 8-12 μM, and mostpreferably about 10 μM Y27632.

42. The method of any one of embodiment 36-41, wherein the seeding stepis carried out 18-30 h prior to step (i).

43. A bioengineered human myocardium (BHM) produced by the methodaccording to any one of embodiment 1-42.

44. The BHM of embodiment 43, which can be paced at multiple frequenciesof up to at least 3 Hz.

45. The BHM of embodiment 43 or 44, which exhibits an increased twitchtension in response to increased resting length and resting tension.

46. The BHM of any one of embodiment 43-45, which exhibits a calciumEC₅₀ higher than 0.2 mM.

47. The BHM of any one of embodiment 43-46, which exhibits a twitchtension of more than 200 μN.

48. The BHM of any one of embodiment 43-47, which exhibits an inotropicresponse to 1 μM isoprenaline of more than 40 μN under paced conditionsat 0.6 mM calcium, preferably more than 45 μN, more preferably more than50 μN.

49. The BHM of any one of embodiment 43-48, which comprisescardiomyocytes and CD90⁺ stromal cells.

50. The BHM of any one of embodiment 43-49, which can be maintained forat least 62 days.

51. Use of the bioengineered human myocardium (BHM) according to any oneof embodiment 43-50 in an in vitro-model for drug toxicity screening.

52. Use of the bioengineered human myocardium (BHM) according to any oneof embodiment 43-50 in an in vitro method for testing of cardiacfunction modulation by pharmacological candidate agents.

53. Use of the bioengineered human myocardium (BHM) according to any oneof embodiment 43-50 as a research tool.

54. The bioengineered human myocardium (BHM) according to any one ofembodiment 43-50 for use in medicine.

55. The bioengineered human myocardium (BHM) according to any one ofembodiment 43-50 for use in heart repair.

DESCRIPTION OF THE FIGURES

FIG. 1: Optimization of early cardiac differentiation for robust andefficient cardiac differentiation. (A) Schematic of the developedcardiac differentiation protocol. (B,C) Effect of FGF-2 addition oncardiac differentiation in 2D culture. (D,E) Effect of varying BMP4concentration on cardiac differentiation in 2D culture whilst CHIR ispresent. (F,G) Effect of removing each factor individually from the 2Dcardiac differentiation protocol, all factors were added daily from days0-3 except IWP4 which was added every 2-3 days from days 3-13. (H,I,J)Assay for the presence of contaminating cell types using qPCR. (K)Immuno-staining for cardiomyocyte markers. (L) Flow cytometry forcardiomyocytes (n=6 experiments). (M) Immuno-staining for stromal cellmarkers. (N) Flow cytometry for stromal cells (n=6 experiments). Alldata is n=3 experiments unless otherwise noted. qPCR data (MESP-1, OCT4,SOX17 and NEUROD1) is normalized to GAPDH. * Indicates statisticallysignificant difference (P<0.05) using ANOVA with Tukey's MultipleComparison Post Hoc test. ** Indicates statistically significantdifference from samples supplemented with no factors. *** Indicatesstatistically significant difference from samples supplemented with nofactors, all factors minus BMP4, all factors minus ACT-A and all factorsminus IWP-4.

FIG. 2: BHM can be formed directly from hPSC. (A) BHM at 22 days ofdifferentiation. (B) Whole-mount immunostaining. (C) Isometric twitchtension (force of contraction) in response to varying calciumconcentration, n=7 from 4 experiments. (D) Flow cytometry profiles ofpluripotency (TRA-1-60/OCT4) and cardiac markers (α-actinin), n=3-4experiments. (E) Flow cytometry of stromal cell markers at day 22, n=3experiments. (F) qPCR expression profiles of markers for pluripotency,mesodermal differentiation, and cardiac differentiation; data isnormalized to GAPDH expression, n=3 experiments. * Indicatesstatistically significant difference (P<0.05) compared to −1 days usingANOVA with Tukey's Multiple Comparison Post Hoc test.

FIG. 3: Optimization of BHM culture condition reveals differentparameters respond specifically to different stimuli. (A) ASC-2-Psupplementation improves BHM, isometric twitch tension (force ofcontraction) in response to varying calcium concentration, n=8-9 from 3experiments, * indicates statistically significant difference (P<0.05)compared to control using two-way ANOVA with Bonferroni post hoc tests.(B) Flow cytometry analysis of ASC-2-P experiments for cardiomyocyte andstromal cell markers, n=7-8 from 3 experiments. (C) Mechanicalstimulation improves BHM function, isometric twitch tension (force ofcontraction) in response to varying calcium concentration, n=9-11 from 4experiments, * indicates statistically significant difference (P<0.05)for both mechanical stimulation regimes compared to control usingtwo-way ANOVA Bonferroni post hoc tests. (D) Mechanical stimulationdevices. (E) Whole-mount immuno-staining of BHM under control andmechanical stimulation regimes. (F) Growth factors (FGF2:10 ng/mL andTGFb1: 1 ng/mL) added during cardiac maturation regulate BHM function,isometric twitch tension (force of contraction) in response to varyingcalcium concentration, n=9-11 from 4 experiments. (G) Cardiomyocyte cellsize analysis for the growth factor experiments using flow cytometry,n=6 from 3 experiments, * indicates statistically significant difference(P<0.05) compared to control using Student's t-test. (H) qPCR expressionof β-MHC/α-MHC ratio for the growth factor experiments, n=3-6experiments, * indicates statistically significant difference (P<0.05)compared to control using ANOVA with Tukey's Multiple Comparison PostHoc test. (I) qPCR expression of ANP and Sk Act for the growth factorexperiments, n=3 experiments. (J) Adjusting calcium to 1.2 mmol/L duringcariac maturation improves BHM function, isometric twitch tension (forceof contraction) in response to varying calcium concentration, n=10-11from 4 experiments, * indicates statistically significant difference(P<0.05) compared to control using two-way ANOVA Bonferroni post hoctests. (K) resting tension of BHM for the calcium experiments, n=10-11from 4 experiments. (L) Elastic modulus of BHM for the calciumexperiments, n=10-11 from 4 experiments, * indicates statisticallysignificant difference (P<0.05) compared to control using ANOVA withTukey's Multiple Comparison Post Hoc test.

FIG. 4: BHM produced using the optimized protocol exhibit in-vivo-likeproperties. (A) BHM can be paced electrically at various rates. (B) BHMrespond with increased twitch tension (force of contraction) toincreased length (Frank-Starling mechanism). (C) Twitch tension (forceof contraction) comparison of 22 day old (from previous data) and 29-30day old BHM, n=11 from 4 experiments for 22 days and n=7 from 2experiments for 29-30 days. (D) Inotropic response of 22 day old BHM toisoprenaline (1 μmol/L) under paced conditions at 0.6 mM calcium. (E)Inotropic response of 29-30 day old BHM to isoprenaline (1 μmol/L) underpaced conditions at 0.6 mM calcium. (F) Comparison of inotropic responseto isoprenaline (1 μmol/L) with age, n=11 from 4 experiments for 22 daysand n=7 from 2 experiments for 29-30 days, * indicates statisticallysignificant difference (p<0.05) using Student's t-test.

FIG. 5: The BHM protocol can be used in all tested PSC lines. (A,B,C)Data from HES3-BHM; (D,E,F) Data from hIPS-G1-BHM. (A,D) Isometrictwitch tension (force of contraction) in response to varying calciumconcentration, n=4 for each line. (B,E) Whole-mount immuno-staining.(C,F) Flow cytometry analysis of cardimoycytes and stromal cells, n=3per line.

FIG. 6: 2D cardiac differentiation of multiple hPSC lines. Flowcytometry analysis of cardiac (α-actinin, SIRPA) and stromal cellmarkers (PDGFRα, α-SMA, collagen I).

FIG. 7: The BHM can be used to model developmental processes, using BMPsignalling inhibition as an example. (A) qPCR analysis of multiplemarkers at day 13 of BHM formation. (B) Cell cycle analysis using flowcytometry gating on α-actinin+cells. (C) Cardiomyocyte number per BHM.(D) Isometric twitch tension (force of contraction) in response tovarying calcium concentrations. * indicates statistically significantdifference (P<0.05) compared to control using A+B) Student's t-test(n=3-4) or D) two-way ANOVA Sidak's Multiple Comparison post test.

FIG. 8: Summary of protocols used for 2D cardiac differentiation and BHMformation. (A) Protocol used for experiments shown in FIG. 1. (B)Protocol used for experiments shown in FIG. 2. (C) Protocol used forexperiments shown in FIG. 3—addition of ascorbic acid. (D) Protocol usedfor experiments shown in FIG. 3—mechanical stimulation and growthfactors. (E) Protocol used for experiments shown in FIG. 3—addition ofcalcium. (F) Protocol used for experiments shown in FIG. 5. (G) Protocolused for experiments shown in FIG. 6. (H) Protocol used for experimentsshown in FIG. 7.

FIG. 9: Custom-made supplement to replace B27®. (A) Contractile force ofBHMs (hES2) made with B27® or custom-made supplement (CMS) at 2 mMextracellular calcium, n=2/group. (B) Total CM number in BHM made withB27® or custom-made supplement (CMS), n=2/group.

The following examples are meant to further illustrate, but not limitthe invention. The examples comprise various technical features, and itwill be appreciated that the invention also relates to combinations ofthe technical features presented in this exemplifying section.

FIG. 10: Supplementation of the culture medium with TGFβ-1 during thecardiac maturation phase enhances contractile function of BHM (FOC:force of contraction) in a concentration dependent manner (0.3-10 ng/mltested; n=5-7 BHMs/condition).

EXAMPLES

Cardiac Differentiation Requires Optimization of Early Cardiac MesodermInduction

It has been demonstrated that non-myocyte cell fractions or stromalcells are essential for the function of engineered heart tissues. Forthis reason a cardiac differentiation protocol was firstly requiredwhich consistently produced cardiomyocytes and fibroblasts/stromalcells. The inventors optimized their cardiac differentiation protocol(FIG. 1a ) for both yield and consistency, based on a previouslypublished serum-free 2D hPSC differentiation protocol (Hudson et al.Stem Cells Dev 21, 1513-1523 (2012)). It was reasoned that robustnessand efficiency could be enhanced if WNT activity was stabilized duringthe mesoderm induction phase. As surrogate marker for mesoderm inductionMESP1 expression was analysed by qPCR on culture day 3; this wasfollowed by flow cytometry for α-actinin (cardiomyocyte marker) at day16, which were found correlated very well with the amount of beatingactivity. The most important steps for the progression from thepreviously published protocol to the new protocol are summarised in FIG.1.

The earlier protocol used cardiac mesoderm induction with BMP4 andActivin-A for the first 3 days followed by cardiac specification using aWNT inhibitor IWP4 (Hudson et al Stem Cells Dev 21, 1513-1523 (2012)).Consistent with the essential requirement of FGF2 for early mesodermformation in recent studies using hPSCs and in vivo development, theaddition of 5 ng/ml FGF2 in during the first 3 days of differentiationresulted in a trend to increased MESP1 expression (FIG. 1b ) andsubsequently a trend to increased α-actinin (FIG. 1c ). However, it wasenforced WNT signalling that helped improve consistency anddifferentiation, consistent with its essential role in induction ofearly mesoderm. To enforce WNT signalling CHIR99021 was used, a smallmolecule inhibitor of GSK3β which induces WNT signalling even in thepresence of canonical WNT inhibitors. CHIR alone or with thedifferentiation factors used in FIG. 1b,c was unable to induce anybeating activity. When the BMP4 concentration was varied, optimal andconsistent induction of MESP1 (FIG. 1d ) and α-actinin expression (FIG.1e ) was found at a BMP4 concentration of 5 ng/ml. Each factor from thedifferentiation protocol was then individually removed to demonstrateits requirement in the efficient and consistent induction of MESP1 (FIG.1f ) and subsequently α-actinin expression (FIG. 1g ).

In order to form BHM it is also important that stromal cell populationsare present. It was therefore investigated whether stromal cells orother potentially contaminating cell types were present in the optimizeddifferentiation protocol. Very low levels of potentially contaminatingcell populations were found, analysed using qPCR for hPSCs (OCT4 alsoknown as POU5F1) (FIG. 1h ), endoderm (SOX17) (FIG. 1i ), neural(NEUROD1) (FIG. 1j ), and early mesoderm (MESP1) (data not shown). Veryhigh expression of NKX2-5 and β-MHC (also known as MYH7) were found inthe cardiac differentiation culture of the invention compared to theother conditions (with no factors or without IWP4, data not shown).Additionally it was found that both cardiomyocytes (FIG. 1k-l ) anddifferent stromal cell types, including: α-smooth muscle actin positivecells (α-SMA⁺), collagen I positive cells (COLI⁺) cells, and α-SMA⁺COLI⁺cells were present (FIG. 1m,n ). Together this data suggests that thecardiac differentiation protocol of the invention efficiently producescardiomyocytes with the rest of the cells being predominantly stromalcells, thus providing the required cell composition for tissueengineering applications.

Directed Formation of BHM

After optimization of the cardiac differentiation protocol thehypothesis was tested whether one could form BHM directly from hPSCs.The new serum-free cardiac differentiation protocol (FIG. 1) was usedwith an additional maturation step where the rings were removed from themolds and put onto static stretchers (+10% of slack length) in mediumcontaining 5 ng/ml FGF2 and 200 μM ascorbate-2-phosphate (ASC-2-P). Itwas found that this protocol was effective in forming BHM (FIG. 2a ).The BHM started to contract spontaneously in different areas by day 13and by day 15-17 the contractions became synchronous and rhythmic, andcontinued until analysis at day 22 (data not shown). The cardiomyocytesin the BHM had an elongated and striated morphology (FIG. 2b ) and theBHM could be electrically paced and had a measurable force ofcontraction with responsiveness to calcium concentration (FIG. 2c ).

The development of the BHM followed known developmental pathways. ThehPSCs were largely differentiated by day 3 indicated by a decrease inTRA-1-60⁺/OCT4⁺ cells and OCT4 expression (FIG. 2d,f ) and a concomitantexpression of early mesodermal markers MIXL1 and MESP1 (FIG. 2f ). Byday 8 there was a loss of MIXL1 and MESP1 expression (FIG. 2f ), with aconcurrent increase in α-actinin⁺ cells (FIG. 2d ) and the expression ofcardiomyocyte progenitor cell markers (FIG. 2f ). There were peaks inthe expression of multiple transcription factors involved incardiogenesis, including: TBX5(peak at day 13), ISL1 (peak at day 8),and NKX2-5(peak at day 13) (FIG. 2f ). This was followed by theexpression of more mature cardiac markers α-MHC (also known as MYH6),β-MHC, ANP (also known as NPPA), and MLC2v (also known as MYL2) (FIG. 2f). Interestingly, the expression of α-MHC peaked at day 13, followed bya large increase in β-MHC by day 22 and hence an increased β-MHC/α-MHCratio (FIG. 2f ). Furthermore, there was little expression of endodermaland neural markers at day 22 (FIG. 2f ). Together this data suggests notonly that the development of the BHM followed known developmentalpathways, but also that cardiac maturation occurs as indicated by thedrop in progenitor gene expression, increased β-MHC expression andβ-MHC/α-MHC expression ratio, and increased expression of MLC2v(indicates maturity, see Tiburcy et al. Circ Res 109, 1105-1114 (2011)).Additionally, at day 22 it was found that the BHM was comprised of 30±6%(n=4 experiments) cardiomyocytes and a large proportion of stromal cells(FIG. 2d ). Representative flow cytometry plots are shown in FIG. 5.

Optimization of BHM Functionality

While the BHM protocol outlined in FIG. 2 represents the first entirelyserum-free process of hPSC expansion and formation bioengineeredmyocardium, the inventors hypothesized that optimization could generatetissues with higher functionality and greater consistency. For theseexperiments contractile strength (twitch tension/force of contraction)was used as a primary determinant of function as twitch tension dependson a wide variety of myocardial properties including: cardiomyocytenumber and phenotype, fibroblast number and phenotype, tissueconnectivity, ECM composition, cell-cell connectivity and ECM-cellconnectivity. As secondary factors the inventors used, 1) restingtension, because it reflects stroma cell function and extracellularmatrix biology, 2) cardiomyocyte size, because of stimuli such aspharmacological stimuli, and 3) cell composition (cardiomyocytes:stromacells), which is an important determinant for contractile performance.The parameters which changed are shown in FIG. 3.

ASC-2-P Enhances BHM Functionality

Ascorbate (vitamin C) plays a major role in the proper synthesis ofcollagen and is an antioxidant. It was therefore hypothesized thatascorbate (in the more stable form of ASC-2-P) supplemented during earlyBHM culture, days 0-13 (it was already added during days 13-22 in FIG.2), would have a positive influence on BHM functionality given theimportance of collagens during development. It was found that ASC-2-Psignificantly improved BHM twitch tension/force of contraction (FIG. 3a) and induced a trend to increased cardiomyocyte fraction with no changein the stromal cell fraction (FIG. 3b ). It was also found thatsupplementation of ASC-2-P during the entire differentiation protocolimproved differentiation in the inventor's 2D protocol by significantlyincreasing the number of cells without changes in cardiacdifferentiation efficiency (data not shown). Therefore, in both 2D andBHM formats, ASC-2-P may have increased cell survival and/or progenitorproliferation as proposed in a recent study (Cao et al. Cell Res 22,219-236 (2012)).

BHM Function is Dependent on Mechanical Stimulation Regime

Next, it was assessed how static stretch and dynamic mechanicalstimulation influenced BHM function. The devices used for static stretchand dynamic mechanical stimulation are shown in FIG. 3d . Both staticstretch and dynamic mechanical stimulation significantly increasedtwitch tension/force of contraction of the BHM and both the mechanicalstimulation regimes resulted in a similar BHM twitch tension (FIG. 3c ).Both mechanical stimulation regimes improved the morphology of thecardiomyocytes in the BHM, causing compact, elongated and striatedmuscle bundles to form (FIG. 3d ). Dynamic mechanical stimulation werepreferred to static stretch, because it facilitates auxotoniccontractions (Zimmermann et al, Nat Med 12, 452-458 (2006)).

Cardiomyocyte Properties are Dependant on Exogenous Growth Factors

There was a trend to decreased twitch tension/force of contraction whenadding FGF2 and a trend to increased twitch tension when adding TGFβ1(FIG. 3f ). Therefore, in the previous experiments where FGF2 was addedduring cardiac maturation, this may have actually been detrimental BHMfunction. It was found that both FGF2 and TGFβ1 induced an increase incardiomyocyte size (FIG. 3g ). The addition of TGFβ1 resulted in a moremature β-MHC/β-MHC expression ratio (human heart=9) (FIG. 3h ) whilepathological hypertrophy markers ANP (also known as NPPA) was decreased(FIG. 3i ). The addition of FGF2 did not change β-MHC/α-MHC expressionratio (FIG. 3h ), but induced (variably) pathological hypertrophy markerANP (FIG. 3i ). Together this indicates both factors induce hypertrophy,which is consistent with in vivo results. However, FGF2 may beconsidered as an inducer of pathological hypertrophy, while TGFβ1 may beconsidered as an inducer of physiological hypertrophy.

In a further set of experiments the inventors investigated ifsupplementation of the culture medium with increasing TGFβ-1 duringcardiac maturation phase has an influence in contractile function ofBHM. The inventors observed an enhancement of contractile function ofBHM in a concentration dependent manner (FIG. 10).

In the earlier experiments the inventors found a large reduction inα-smooth muscle actin and collagen I positive cells in the BHM whencompared with the 2D protocol (FIG. 1n vs. FIG. 2e ). This may be areflection of subtle differences in myofibroblast/fibroblastdifferentiation in 2D and BHM cultures. To more homogeneously detectcardiac-fibroblast-like populations in hPSC-derived 2D and BHM cardiacdifferentiation cultures antibodies against the canonical CD90 (alsoknown as THY1) were employed in subsequent experiments.

Adjusting Extracellular Calcium to Physiological concentrations ImprovedBHM Function

Calcium concentration is tightly regulated in human serum withphysiological calcium concentrations of 2.25-2.75 mM and 1.0-1.2 mM, fortotal calcium and ionized calcium, respectively. Because theconcentration of calcium in RPMI medium is quite low (0.42 mM) comparedto physiological calcium it was therefore assessed whether adjustment offree calcium concentration improved both BHM maturation andfunctionality. Adjustment of calcium to 1.2 mM (using a 0.2 M CaCl₂solution) greatly increased the twitch tension of the BHM (FIG. 3j ).Moreover, an increase in resting tension (FIG. 3k ) and elastic modulus(FIG. 3l ) was observed. Optimal elastic modulus has been demonstratedto improve the mechanical work done by cardiomyocytes duringcontractions which may increase contraction force. The mechanism behindthe increased elastic modulus in response to increased calcium iscurrently unknown. Importantly, it should be noted that there were nochanges in calcium handling proteins that were assessed using qPCR(CASQ2, PLN, ATP2A2, and RYR2, data not shown, n=3 experiments).

BHM Produced Using the Optimized Protocol Display In-Vivo LikeProperties BHM contacted spontaneously and coherently, and could bepaced electrically at multiple frequencies up to at least 3 Hz (FIG. 4a), covering the range of beating frequency observed during heartdevelopment. BHM also increased twitch tension (force of contraction) inresponse to increased resting length (and resting tension) consistentwith the Frank-Starling mechanism (FIG. 4b ). With increased culturetime no change in BHM twitch tension compared to the previous data wasfound, but an increase in calcium EC₅₀ from 0.2 to 0.7 mmol/L wasobserved (FIG. 4c ). Extended culture also resuited in an increasedinotropic response to 1 μM isoprenaline indicating improved maturityover traditional culture formats where no increase has been observed(FIG. 4d-f ). Using whole-mount immunostaining, it was found that theBHM also has both stromal cells and endothelial cells present in themuscle bundles (data not shown). Importantly BHM culture could also bemaintained for long term (until at least day 63) under the maturationconditions with an observed improvement in morphological appearance(data not shown).

The Unmodified BHM Protocol Works on Multiple Human Pluripotent StemCell Lines

Next, it was demonstrated that the optimized BHM (FIGS. 5) and 2D (FIG.6) protocols work on multiple hPSC lines. For these analyses HES2, HES3and hIPS-G1 lines (reprogrammed dental fibroblasts using vector-freeCytotune reprogramming kit) were used. It is important to note that forboth the 2D and BHM protocols it was found that modification of theseeding cell number or the use of a Rho-associated protein kinaseinhibitor (10 μM, Y-27632) was required to achieve similar celldensities in all lines after the initial 24 h seeding phase (data notshown). When using the required seeding protocol for a particular cellline the 2D and BHM protocols could be used un-modified for those lines.

It was found that both the HES3 line and the hIPS line produced BHM witha lower twitch tension (FIG. 5a,d ) compared to the HES2 line (FIG. 3j). However, both the HES3 and hIPS BHM had similar morphologies (FIG.5b,e ). The cardiomyocyte fractions for the HES3 BHM was similar (FIG.5c ) and for the hIPS BHM was lower (FIG. 5f ), compared to the HES2BHM. The decreased functionality in the BHM from these lines is mostlikely from a lower cell number in the HES3 BHM (0.50±0.03×10⁶ cells,n=3) and hIPS BHM (0.55±0.05×10⁶ cells, n=3), compared to the HES2 BHM(0.74±0.13×10⁶ cells, n=6 from 3 experiments). Therefore, care must betaken to exclude differences caused by changes in cell number andcomposition when assessing different cell lines rather than differenttreatments on the same line.

BHM as a Developmental Model Reveals that BMP Signalling is Required forTerminal Differentiation of Human Cardiomyocytes

Inhibition of BMP signalling is embryonically lethal even when effectsare restricted (or at least partially restricted) to the developingheart using CRE driven by various genes (for a review please seeKruithof et al. Differentiation 84, 89-102 (2012)). In these studiesthere have been multiple processes ascribed to BMP signalling includingstructural defects, myocardial properties including trabeculae structureand wall thickness, and cell phenotype including dysregulation ofprogenitor genes and reduced epithelial-to-mesenchymal transformation(EMT). In order to determine the effect of BMP signalling purely onmyocardial development without systemic influences and anatomicallimitations, BHM was therefore deemed to be a good model system.

In these experiments 2 μmol/L of the BMP receptor signalling inhibitordorsomorphin were added with each medium exchange from day 6 onwards. Atday 13 the dorsomorphin treated BHM failed to down regulate ISL1 whilethe expression of other more mature cardiac markers NKX2-5 and α-MHCwere not altered (FIG. 7a ). When investigating EMT associated genes, itwas found that there was no alteration in CDH1, CHD2, SNAIL1 or TGFβ2expression indicating that EMT or factors regulating EMT were notaltered in BHM after 13 days (FIG. 7a ). Using flow cytometry after 22days it was found that there were more cardiomyocytes in the active cellcycle in the dorsomorphin treated group (FIG. 7b ). However, it wasfound that treatment with dorsomorphin did not alter the cardiomyocytenumber (FIG. 7c ). It was also found that the total cell number per BHMand the fractions of cardiomyocytes (α-actinin⁺) and stromal cells(CD90⁺) did not change in the dorsomorphin treated group (data notshown). Despite these similarities there was a large reduction in twitchtension/force of contraction to 47% of the control group in thedorsomorphin treated BHM (FIG. 7d ). Interestingly, there was neither achange in BHM responsiveness to isoprenaline nor a change incardiomyocyte size in the dorsomorphin treated BHM (data not shown).

As increased cell cycle activity did not lead to increased cardiomyocytenumber and reduced twitch tension, it was calculated whether oxygenconcentration may limit cardiomyocyte number in the BHM. To determine ifthis was the case, mathematically modelled oxygen diffusion profileswere based on models reported in the literature and the parameters inthe different BHM conditions (cell number, cardiomyocyte fraction andsize). It was found that when using the parameters for the control BHMthere was no hypoxic region, even if the number of cardiomyocytesincreased to 125% (data not shown).

Together the present data suggests that BMP inhibition usingdorsomorphin results in an increased proliferative state, a result whichis consistent with mouse in vivo experiments. However, there was noincrease in cardiomyocyte number indicating that either there isincreased apoptosis or the cardioymocytes are bi-nucleating. Regardlessof the mechanism, inhibition of BMP signalling resulted reproducibly ina tissue phenotype that produces less contractile force (and also lowerforce per cardiomyocyte) and inferior myocardial tissue.

Custom-made Supplement to Replace B27®

BHMs were generated from undifferentiated hESC under serum-freeconditions according to standard BHM protocol. The standard protocolincludes B27® supplement. In this experiment B27® supplement wasreplaced by a defined, custom-made supplement (CMS, Table 4).

The results show that B27® can be replaced by CMS. The force is similar,also the number of cardiomyocytes generated within the BHM is comparable(FIG. 9).

Conclusion

Using directed differentiation of PSCs in collagen I hydrogels thepresent application demonstrates that it is possible to guide BHMassembly under serum-free conditions. BHM has multiple applicationsincluding pharmacological studies, study of developmental processes,heart maturation processes and also potential regenerative applications.

In these examples, the robustness of the newly developed protocol wasdemonstrated by using multiple culture formats and multiple lines fordifferentiation. It will however be noted that while theexperiment-experiment differentiation efficiency was consistent, theefficiency did vary when different batches of reagents were used. It istherefore prudent to establish strict reagent quality control in orderto produce BHM with consistent and defined properties for both in vitroand potential therapeutic applications.

Methods

PSC Culture

HES2-ROSA26-RFP (Irion et al. Nat Biotechnol 25, 1477-1482 (2007)) cellswere obtained from Gordon Keller and HES3 cells were obtained fromEmbryonic Stem Cell International (ESI, Singapore). hIPS were generatedfrom human gingiva biopsy-derived fibroblasts using the CytotuneReprogramming Kit (Applied Biosystems) following the manufacturer'sinstructions.

For IPS generation, 6 days after viral transduction fibroblasts wereplated on irradiated mouse embryonic fibroblasts in fibroblast medium(DMEM high glucose, 2 mmol/L glutamine, 10% FBS (PAA), 100 IU/mlPenicillin, 100 μg/ml Streptomycin, all Gibco except where indicated).The next day the medium was exchanged to PSC-medium (Knock-out DMEM(Gibco) supplemented with 20% Knock-Out Serum Replacement (KSR, Gibco),2 mmol/L glutamine, 100 IU/ml Penicillin, 100 μg/ml Streptomycin, 1%non-essential amino acids (Gibco), and 10 ng/mL FGF2 (Miltenyi Biotec)).Emerging iPS colonies were mechanically picked and expanded by weeklypassaging using 1 mg/ml Collagenase NB6 (Cresent Chemical Company).

For experiments, hPSCs were single cell adapted and cultured onirradiated human foreskin fibroblasts (HFF) in PSC-medium with dailymedium changes and weekly passaging using 3 min TrypLE (Gibco) treatment(Ellerstrom et al., Stem Cells 25, 1690-1696 (2007)). Beforecharacterization or differentiation experiments, the hPSCs were platedon 1:30 Matrigel (Millipore) in PBS (Gibco) coated-plates, at 2.5×10⁴cells/cm² for HES2 or 5×10⁴ cells/cm² for HES3 and hIPS lines, andcultured for 3 days in 1:1 PSC-medium minus FGF-2 and HFF-conditionedmedium (HFF-CM—harvested from 5 day confluent irradiated HFF cultures)with 10 ng/mL FGF2. The hIPS line also received 10 μmol/L Y-27632(Stemgent). hPSCs were harvested for experiments by passaging using 3min TrypLE treatment and then cultured in the appropriate format.

Pluripotent stem cell lines were regularly tested for mycoplasma using atest kit (Lonza) and characterized using standard assays. Pluripotencymarkers were assessed via PCR (endogenous OCT4, SOX2, KLF4, MYC), qPCR(OCT4, NANOG, REX1, DNMT3B) and immunostaining (OCT4, NANOG, TRA-1-60)(Chan et al. Nat Biotechnol 27, 1033-1037 (2009)). Demethylation of theOCT4 promoter was confirmed via bisulfite sequencing (Freberg et al. MolBiol Cell 18, 1543-1553 (2007)). Karyotyping was used to determine ifthere were any genetic abnormalities (Campos et al. J Vis Exp 4 (2009)).Pluripotency was confirmed via teratoma formation in SCID mice via flankinjection of 4-6×10⁶ cells.

Differentiation Medium

For differentiation experiments the hPSCs were then cultured in RPMI1640 supplemented with 1 mmol/L sodium pyruvate, 100 IU Penicillin, 100μg/ml Streptomycin and 2% B27 supplement (SF medium, all Gibco) andvarious factors as indicated. Factors used in this study included:L-ascorbic acid 2 phosphate sesquimagnesium salt hydrate (Sigma), BMP4(R&D Systems), Activin A (R&D Systems), FGF2 (Miltenyl Biotec),dorsomorphin (Stemgent), CHIR99021 (Stemgent), IWP4 (Stemgent), andTGFβ1 (Peprotech).

2D Cardiac Differentiation

Cardiac differentiation was optimized on the HES2 line. HES2 hPSCs wereplated at 5×10⁴ cells/cm² (1×10⁵ cells/cm² for HES3 and hIPS lines) on1:30 Matrigel/PBS coated-plates and cultured in 1:1 PSC-medium minusFGF-2 and HFF-conditioned medium (HFF-CM—harvested from 5 day confluentirradiated HFF cultures) with 10 ng/mL FGF2. For the hIPS line 10 μMY-27632 was added to this medium. After 1 day the cells were rinsed withRPMI medium, then differentiated as indicated in each figure with 0.5 mlof medium in each well of a 24 well plate. The protocol details for eachfigure are outlined in FIG. 8.

BHM Formation

BHM formation was optimized on the HES2 line. HES2 hPSCs were suspended1:1 in PSC-medium minus FGF-2 and HFF-conditioned medium(HFF-CM—harvested from 5 day confluent irradiated HFF cultures) with 10ng/mL FGF2 and mixed with a collagen I hydrogel. For the HES3, and hIPSlines 10 μM Y-27632 was also added to the medium. The collagen I matrixwas formulated with acid-soluabilized bovine collagen I (Devro) with anequi-volume of 2× DMEM (Gibco) and neutralized using 0.1 M sodiumhydroxide. The hPSC/collagen I matrix was formulated to give a finalcollagen I concentration of 1 mg/ml and 5×10⁵ hPSC per 170 μl. For theHES3 and hIPS lines 1×10⁶ and 0.5×10⁶ cells were used respectively per170 μl. For each BHM, 170 μl of the hPSC/collagen I matrix was pipetteinto circular molds (i.d.=4 mm, o.d.=10 mm) fabricated usingpoly(dimethylsiloxane) (Sylgard, Dow Corning). After 10 min of culturein the incubator at 37° C. the collagen gelled and 1.25 ml of 1:1 humanforeskin fibroblast-conditioned medium with 10 ng/ml FGF2 was added perBHM. The following day the BHMs were rinsed with RPMI medium and thendifferentiated as indicated in each figure, with 1.25 ml of medium perBHM. At day 13 the BHM were transferred onto mechanical stimulators asindicated. The protocol details for each figure are outlined in FIG. 8.

Cell Disassociation

2D cultures were disassociated by rinsing with PBS followed byincubation for 1 h in 1 mg/ml collagenase type I (Sigma) with 20% fetalbovine serum (FBS, Applied Biosystems) in PBS. The cells were thencollected in a tube, rinsed with PBS and incubated with 0.25%Trypsin-EDTA (Applied Biosystems) for 5 min followed by rinsing with FBScontaining medium.

For the initial BHM digestion protocol, BHMs were disassociated in 0.025mg/ml Liberase™ (Roche), 30 mM 2,3-butanedione monoxime at 37° C. for 60min in PBS. To preserve cell surface markers, BHM was disassociatedusing the same protocol as for the 2D digestion.

Quantitative PCR (qPCR)

Cells, BHMs or human heart biopsies were harvested and stored at −80° C.until RNA extraction using Trizol following manufacturer's instructions(Applied Biosystems). 1 μg of RNA was then treated with DNAse (Roche)followed by cDNA synthesis using a High Capacity cDNA ReverseTranscription Kit (Applied Biosystems).

qPCR was performed using Fast SYBR Green Master Mix (Applied Biosystems)on a 384-well format AB7900 HT (Applied Biosystems). Gene expression wasnormalized using 2^(−ΔCt) or 2^(−ΔΔCt) using GAPDH as the housekeepinggene which we found to be consistently expressed between conditions inall of our experiments. Primer details are given in below Table 1.

TABLE 1 Gene Size (Acc #) F R Purpose (bp) GAPDH CCTCAAGATCATCA-ATGTTCTGGAGAGCCCCGC qPCR 189 (NM_002046.3) GCAATGCC (SEQ ID(SEQ ID NO: 2) RT-PCR NO: 1) OCT4 CAGTGCCCGAAAC- GGAGACCCAGCAGCCTCA-qPCR 161 (NM_002701, CCACAC (SEQ ID NO: 3) AA (SEQ ID NO: 4) NM_203289,NM_001173531) NANOG CAGAAGGCCTCA- ATTGTT- qPCR 111 (NM_024865)GCACCTAC (SEQ ID CCAGGTCTGGTTGC (SEQ NO: 5) ID NO: 6) REX1CACCGCCTCCCTTGG- TGTTCTGTTCA- qPCR  83 (NM_174900.3) GAATTCAG (SEQ IDCACAGGCTCCAGC (SEQ ID NO: 7) NO: 8) DNMT3B GGCCCAAGTAAAC-ATGCCTGGTGTCTCCCTT- qPCR 168 (NM_175848.1 CTAGCTCGGC (SEQ IDCATGC (SEQ IOD NO: 10) NM_175849.1 NO: 9) NM_006892.3) MIXL1 CCGAGTCCAG-CTCTGACGCCGAGACTTGG qPCR  58 (NM_031944) GATCCAGGTA (SEQ ID(SEQ ID NO: 12) NO: 11) NKX2-5 ACAACTTCGT- GTGGACACTCCCGAGTT- qPCR  82(NM_001166175.1, GAACTTCGGCG (SEQ GCTCT (SEQ ID NO: 14) NM_001166176.1,ID NO: 13) NM_004387.3) TBX5 TCATAACCAAGGCTG- GCCCGTCACAGAC- qPCR 152(NM_000192.3, GAAGG (SEQ ID NO: CATTTAT (SEQ ID NO: 16) NM_080717.2, 15)NM_181486.1, NM_080718.1) ISL1 CGCCTTGCAGAGTGAC GGACTGGCTACCAT- qPCR 147(NM_002202.2) ATAG (SEQ ID NO: 17) GCTGTT (SEQ ID NO: 18) MYH6CTCCTCCTACGCAACTG CGACACCGTCTGGAAG- qPCR  85 (α-MHC) CCG (SEQ ID NO: 19)GATGA (SEQ ID NO: 20) (NM_002471) MYH7 GACCAGATGAAT- GGTGAGGTCGTT- qPCR 63 (β-MHC) GAGCACCG (SEQ ID GACAGAACG (SEQ ID NO: (NM_0002571) NO: 21)22) MLC2v GGCGCCAACTCCAACG ACGTTCA- qPCR 149 (NM_000432.3)TGTT (SEQ ID NO: 23) CTCGCCCAAGGGC (SEQ ID NO: 24)ACTA1 (Skeletal actin) ACCCAGATCATGTTT- TCATAAATGGGCACGTT- qPCR 143(NM_001100.3) GAGACC (SEQ ID NO: GTG (SEQ ID NO: 26) 25) NPPA (ANP)TCTGCCCTCCTAAAAA TGTCCTCCCTGGCTGTTAT qPCR 156 (NM_006172.3)GCAA (SEQ ID NO: 27) C (SEQ ID NO: 28) CASQ2 TCTTGCAGGGCAGAAGGGACCTGGGCCACAAGCTC qPCR 205 (NM_001232.3) AGGGG (SEQ ID NO:AA (SEQ ID NO: 30) 29 NEUROD1 AGCCAC- GCGTGCCTCTAATCATGAA qPCR 143(NM_002500.3) GGATCAATCTTCTC A (SEQ ID NO: 32) (SEQ ID NO: 31)CDH1 (ECAD) GAACGCATT- ATTCGGGCTTGTT- qPCR 118 (NM_004360.3)GCCACATACAC (SEQ GTCATTC (SEQ ID NO: 34) ID NO: 33) CDH2 (NCAD)CCTGGAACGCAGTG- TGGTTTGACCACGGTGAC- qPCR 104 (NM_001792.3)TACAGA (SEQ ID NO: TA (SEQ ID NO: 36) 35) SNAIL1 AGCGAGCTGCAG- GGACAGAG-qPCR 136 (NM_005985.3) GACTCTAA (SEQ ID TCCCAGATGAGC (SEQ ID NO: 37)NO: 38) MESP1¹ AGCCCAAGT- AAGGAACCACTT- qPCR  82 (NM_018670.3)GACAAGGGACAACT CGAAGGTGCTGA (SEQ ID (SEQ ID NO: 39) NO: 40) SOX17¹AGGAAATCCTCA- CCCAAACTGTTCA- qPCR 111 (NM_022454.3) GACTCCTGGGTT (SEQAGTGGCAGACA (SEQ ID ID NO: 41) NO: 42) ATP2A2 (SERCA2) ACCTCATCTCGTCCAATGTCACCAGATTGACCCAG qPCR 110 (NM_001681.3 CGTC (SEQ ID NO: 43)A (SEQ ID NO: 44) NM_170665.3) PLN ACAGCTGCCAAGGCTA TCCATGATACCAGCAGGACqPCR 114 (NM_002667.3) CCTA (SEQ ID NO: 45) A (SEQ ID NO: 46) RYR2TGCAAGACTCACCGAA CCACCCAGACATTAGCAGG qPCR 125 (NM_001035.2)GATG (SEQ ID NO: 47) T (SEQ ID NO: 48) COL1A1 GTGCTAAAGGTGCCAAACCAGGTTCACCGCTGTTA qPCR 128 (NM 000088.3) TGGT (SEQ ID NO: 49)C (SEQ ID NO: 50) COL3A1 CCAGGAGCTAACGGTC CAGGGTTTCCATCTCTTCC qPCR 103(NM_000090.3) TCAG (SEQ ID NO: 51) A (SEQ ID NO: 52) COL5A1GACACCTCCAACTCCT AGTGAACTCCCCCTCCAAG qPCR  72 (NM_000093.3)CCAA (SEQ ID NO: 53) T (SEQ ID NO: 54) COL4A1 GTTGGTCTACCGGGACGTTCACCTCTGATCCCCTG qPCR 145 NM_001845.4) TCAA (SEQ ID NO: 55)A (SQ ID NO: 56) LAMC1 GTGAGAGGTGCCGAGA GTGCTTAGAGAGCCCACAG qPCR  88(NM_002293.3) GAAC (SEQ ID NO: 57) G (SEQ ID NO: 58) TGFB2CGAACCCAAAGGG- TAAGCTCAGGAC- qPCR  91 (NM_001135599.2,TACAATG (SEQ ID NO: CCTGCTGT (SEQ ID NO: NM_0032.38.3) 59 60TTN Ex49-50 (ALL GTAAAAAGAGCTGCCC GCTAGGTGGCCCAGTGCTA qPCR  68 TTN)²CAGTGA (SEQ ID NO: CT (SEQ ID NO: 62) (NM_001267550.1 61) NM_001256850.1NM_133437.3 NM_133432.3 NM_003319.4) TTN Ex107-108 CAGCAGAACTCAGAATATCAAAGGACACTTCACAC qPCR 110 (N2BA)² CGA (SEQ ID NO: 63)TC (SEQ ID NO: 64) (NM_001267550.1 NM_001256850.1 NM_133378.4)TTN Ex50-219 (N2B)² CCAATGAGTATGGCAG TACGTTCCGGAAGTAATTT qPCR  93(NM_133437.3 TGTCA (SEQ ID NO: GC (SEQ ID NO: 66) NM_133432.3 65)NM_003319.4) endoOCT4³ CCTCACTTCA- CAGGTTTTCTTT- RT-PCR 164 (NM_002701,CTGCACTGTA (SEQ ID CCCTAGCT (SEQ ID NO: NM_203289, NO: 67) 68)NM_001173531) endoSOX2³ CCCAGCAGACTTCA- CCTCCCATTT- RT-PCR 151(NM_003106.3) CATGT (SEQ ID NO: CCCTCGTTTT (SEQ ID NO: 69 70 endoKLF4³GATGAACTGACCA- GTGGGTCATA- RT-PCR 145 (NM_004235.4) GGCACTA (SEQ ID NO:TCCACTGTCT (SEQ ID NO: 71) 72) endoMYC³ TGCCTCAAATTG- GATTGAAATTCTGTG-RT-PCR 192 (NM_002467.4) GACTTTGG (SEQ ID TAACTGC (SEQ ID NO: 74)NO: 73) SeV GGATCACTAGGTGA- ACCAGACAAGAGTT- RT-PCR 181 TATCGAGCTAAGAGATATGTATC (SEQ ID NO: 75) (SEQ ID NO: 76) BS OCT4-2 TTAGGAAAATGGG-TAC- BS 296 TAGTAGGGATTT (SEQ CCAAAAAACAAATAAAT- ID NO: 77)TATAAAACCT (SEQ ID NO: 78) BS OCT4-44 GGATGTTATTAAGAT- CCTAAACTCCCCTTCA-BS 406 GAAGATAGTTGG (SEQ AAATCTATT (SEQ ID NO: ID NO: 791 80) 1.Kattman, et al. Cell Stem Cell 8, 228-240 (2011). 2. Neagoe, et al.Circulation 106, 1333-1341 (2002). 3. Park, et al. Nature 451, 141-146(2008). 4. Freberg, et al. Mol Biol Cell 18, 1543-1553 (2007).

Immunostaining

Digested cardiac differentiation cells were plated on 0.1% gelatincoated glass coverslips for 24 h in 20% FBS (Gibco) in RPMI 1640supplemented with 1 mmol/L sodium pyruvate, 100 IU/ml Penicillin and 100μg/ml Streptomycin. The cells were then fixed in Histofix (Roti) for 10min at room temperature. The cells were then blocked for 30 min in 5%FBS, 1% bovine serum albumin (Sigma) and 0.5% Triton X-100 (Sigma) inPBS (blocking buffer). The cells were then stained with primaryantibodies in blocking buffer for 90 min followed by secondaryantibodies in blocking buffer and Hoechst for 60 min at room temperature(Table 2). Stained cells were imaged using a Zeiss 710 confocalmicroscope.

TABLE 2 Antibodies and stains (FC—Flow cytometry, IF—Immunofluorescence)Antibody/Stain Supplier Cat No. Marker Dilution IgG₁ RnD Systems MAB002Control 1:100 FC 1:100 IF TRA-1-60-FITC BD Pharmingen 560173Pluripotency 1:100 FC (mouse) 1:100 IF OCT3/4 (rabbit) Santa Cruz Bio-sc-9081 Pluripotency 1:200 FC technology 1:200 IF NANOG (goat) RnDSystems AF1997 Pluripotency 1:20 IF α-smooth muscle Sigma A2547 Stromalcell (used predominately 1:4000 FC actin (mouse) in this study) 1:400 IFα-smooth muscle Abcam ab32575 Stromal cell (used only with 1:50 FC actin(rabbit) α-actinin co-staining) Collagen I Abcam ab34710 Stromal cell1:2000 FC (rabbit) 1:500 IF NKX2-5 (rabbit) Santa Cruz Bio- H-114Cardiomyocyte/cardiac progenitor 1:200 IF technology α-actinin (mouse)Sigma A7811 Cardiomyocyte 1:4000 FC 1:1000 IF Cardiac Troponin IMillipore MAB1691 Cardiomyocyte 1:200 IF (mouse) SIRPA (mouse) Biolegend323802 Cardiomyocyte 1:100 FC PDGFRα (mouse) RnD Systems MAB1264Mesodermal progenitor 1:25 FC CD90 (mouse) RnD Systems MAB2067 Stromalcell 1:125 IF 1:500 FC Donkey anti-goat Invitrogen A-11056 NA 1:400 IFAlexafluor 546 Goat anti-mouse Invitrogen A-11001 NA 1:1000 FCAlexafluor 488 1:400 IF Goat anti-rabbit Invitrogen A-11010 NA 1:1000 FCAlexafluor 546 1:400 IF Phalloidin Invitrogen A22283 NA 1:50 IFAlexafluor 546 Hoechst33342 Invitrogen H3570 NA 1:1000 FC 1:1000 IF

Whole-Mount Immuno-Staining

BHMs were fixed in Histofix for 2-4 h at 4° C. The BHMs were thenstained with primary antibodies for 2-3 days followed by secondaryantibodies and Phalloidin 546/Hoechst for 2-3 days at 4° C. (Table 2).Stained BHMs were imaged using a Zeiss 710 confocal microscope.

Flow Cytometry

Cells were stained either live or fixed using Histofix for 10 min atroom temperature or ethanol. The cells were stained in 5% FBS in PBS(membrane blocking buffer) for cell surface markers (excluding TRA-1-60)and blocking buffer for internal markers. The cells were then stainedwith primary antibodies in blocking buffer for 45 min followed bysecondary antibodies in blocking buffer and Hoechst for 30 min at 4° C.(Table 2). A BD LSRII was used for flow cytometry analysis (BDBiosystems). Live cells populations were gated based on forward-sidescatter profiles; fixed cells populations were gated based on Hoechststaining. BD FACSDiva Software (BD Bioscience) or Cyflologic v1.2.1(Cyflo Ltd) were used for analysis.

Contraction Measurements

Contraction experiments were performed in organ baths at 37° C. andcontinuous bubbling with 5% CO₂/95% O₂ to maintain a physiological pH inTyrode's solution containing (all in mM): 120 NaCl, 1 MgCl₂, 0.2 CaCl₂,5.4 KCl, 22.6 NaHCO₃, 4.2 NaH₂PO₄, 5.6 glucose and 0.56 ascorbate.Calcium was adjusted using a 0.2 M calcium chloride solution. All BHMwere first analysed at 3 Hz with 5 ms square pulses of 200 mA in orderto pace at approximately the embryonic heart rate. BHM were mechanicallystretched at intervals of 125 μm until Lmax, i.e. the tissue length weremaximum twitch tension/force of contraction was recorded in the presenceof maximally inotropically active calcium concentrations (2 mmol/L;Frank-Starling mechanism). Subsequently, BHM were subjected to differentcalcium concentrations (0.2, 0.4, 0.8, 1.2, 1.6, 2.0, 2.4 mM) and thetwitch force recorded. For isoprenaline experiments the calciumconcentration was adjusted to 0.6 mM and subsequently the isoprenalineconcentration was adjusted to 1 μM.

Oxygen Diffusion Profile

The oxygen diffusion profile was generated using numerical analysis of apseudo-steady-state approximation of cylinder diffusion with oxygenconsumption dependence on concentration (Eq. 1). Parameters from theliterature (Brown et al. Biotechnol Bioeng 97, 962-075 (2007)) and asdetermined in previous experiments were used (Table 3). Numericalanalysis and plotting was performed using MATLAB V12 (Mathworks) usingthe solver bvp4c and the Singular Term option.

$\begin{matrix}{0 = {{\frac{- D_{O\; 2}}{r}\lbrack {r\frac{{dC}_{O\; 2}}{dr}} \rbrack} + {V_{\max}\rho_{cardiomyocyte}\mspace{11mu} e^{(\frac{- \alpha}{C_{O\; 2}})}}}} & {{Eq}.\; 1}\end{matrix}$

C_(O2)—oxygen concentration as a function of radial position, r−radialposition in cylinder, D_(O2)—oxygen diffusion constant, V_(max)—maximaloxygen generation rate by cardiomyocytes, ρ_(cardiomyocyte)—density ofcardiomyocytes, α—constant for oxygen generation rate dependence onoxygen concentration.

TABLE 3 Oxygen diffusion model parameters Model Parameter Value D_(O2) 2× 10⁵ cm²/s P_(cardiomyocyte)$\rho_{{cardiomyoc}\mspace{20mu} {yte}} = \frac{{{CellNumber} \cdot {Cardiomyoc}}\mspace{14mu} {ytePurity}}{BHMVolume}$V_(max) −5.44 × 10⁻⁸ nmol/cell/s α Calculated to fit oxygenconsumption-concentration relationship from ⁵; 1.14 μM BoundingConditions${{{At}\mspace{20mu} r} = 0};\; {\frac{\; {dC}_{O\; 2}\mspace{11mu}}{dr} = 0}$At r = R ; C_(O2) = C* R = 0.5 mm; C* = 61 μM

5. Brown, et al. Biotechnol Bioeng 97, 962-975 (2007).

Statistical Analysis

All data is presented as mean±s.e.m. Appropriate statistical analyseswere used for each data set as indicated in the Figure legends usingGraph Pad Prism or Microsoft Excel.

Custom-made Supplement to Replace B27®

TABLE 4 Custom-made supplement (CMS) to replace B27. Final Substanceconcentration 25x Supplier Albumin 5 mg/ml 125 mg/ml Sigma, A9511Transferrin 10 μg/ml 250 μg/ml Sigma, T0665 EthanolamineHCl 2 μg/ml 50μg/ml Sigma, E6133 Sodium selenite 0.032 μg/ml 0.8 μg/ml Sigma, S5361L-CarnitineHCl 4 μg/ml 100 μg/ml Sigma, C0283 Hydrocortisone 1 μg/ml 25μg/ml Sigma, H2270 Fatty acid supplement 0.5 μl/ml 12.5 μl/ml Sigma,F7050 Triiodo-L-thyronine 0.004 μg/ml 0.1 μg/ml Sigma, T 6397 Prepare25x in cell culture-qualified water.

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1. An in vitro bioengineered heart muscle (BHM), wherein the BHM isartificially moulded.
 2. The BHM of claim 1, wherein the BHM has aβ-MHC/α-MHC ratio of more than 0.2 or more than 0.3, but less than theβ-MHC/α-MHC ratio found in adult heart tissue.
 3. The BHM of claim 1,wherein the BHM has a low but still retained expression of progenitorgenes as compared to adult heart tissue.
 4. The BHM of claim 3, whereinthe BHM has a low but still retained expression of ISL-1 as compared toadult heart tissue.
 5. The BHM of claim 1, wherein the BHM does notexhibit blood perfusion.
 6. The BHM of claim 1, wherein the BHM isserum-free.
 7. The BHM of claim 1, wherein the BHM comprisescardiomyocytes and CD90⁺ stromal cells.
 8. The BHM of claim 7, whereinthe BHM is comprised of about 50% cardiomyocytes and the restpredominantly CD90⁺ stromal cells.
 9. The BHM of claim 1, wherein theBHM is made of cells of primate origin.
 10. The BHM of claim 1, whereinthe BHM is made of cells of human origin.
 11. The BHM of claim 1,wherein the BHM is moulded in a collagen matrix, wherein the collagen isselected from the group consisting of collagen type I, collagen typeIII, collagen type V, and a mixture thereof.
 12. The BHM of claim 11,wherein the BHM is moulded in a collagen matrix consisting of at least90% collagen type I.
 13. The BHM of claim 11, wherein the collagenmatrix further comprises one or more extracellular matrix componentsselected from the group consisting of elastin, laminin, entactin,nidogen, proteoglycan, and fibronectin.
 14. The BHM of claim 11, whereinthe collagen is of human origin, bovine origin, porcine origin, ormarine origin.
 15. The BHM of claim 11, wherein the collagen is of humanorigin.
 16. The BHM of claim 11, wherein the collagen is of algae originor fish origin.
 17. The BHM of claim 11, wherein the collagen isrecombinant collagen.
 18. The BHM of claim 1, wherein the BHM has aring-shaped form.
 19. The BHM of claim 1, wherein the BHM is capable ofbeing paced at multiple frequencies up to at least 3 Hz.
 20. The BHM ofclaim 1, wherein the BHM exhibits an increased twitch tension inresponse to increased resting length and resting tension.
 21. The BHM ofclaim 1, wherein the BHM exhibits a calcium EC₅₀ higher than 0.2 mM. 22.The BHM of claim 21, wherein the BHM exhibits a calcium EC₅₀ in therange of 0.2-8 mM.
 23. The BHM of claim 1, wherein the BHM exhibits atwitch tension of more than 200 μN.
 24. The BHM of claim 1, wherein theBHM exhibits an inotropic response to 1 μM isoprenaline of more than 40μN under paced conditions at 0.6 mM calcium.
 25. The BHM of claim 24,wherein the BHM exhibits an inotropic response to 1 μM isoprenaline ofmore than 45 μN under paced conditions at 0.6 mM calcium.
 26. The BHM ofclaim 24, wherein the BHM exhibits an inotropic response to 1 μMisoprenaline of more than 50 μN under paced conditions at 0.6 mMcalcium.
 27. The BHM of claim 1, wherein the BHM can be maintained forat least 62 days.
 28. A method for screening drug toxicity, comprisingthe step of contacting a BHM according to claim 1 with a drug to bescreened.
 29. A method for testing of cardiac function modulation,comprising the step of contacting a BHM according to claim 1 with apharmacological candidate agent.
 30. A method for heart repair,comprising the step of incorporating the BHM according to claim 1 into aheart of a patient in need thereof.